Problem:
Score : 400 points

Problem Statement

Takahashi has N sticks that are distinguishable from each other. The length of the i-th stick is L_i.

He is going to form a triangle using three of these sticks. Let a, b, and c be the lengths of the three sticks used. Here, all of the following conditions must be satisfied:

a < b + c

b < c + a

c < a + b

How many different triangles can be formed? Two triangles are considered different when there is a stick used in only one of them.

Constraints

All values in input are integers.

3 \leq N \leq 2 \times 10^3

1 \leq L_i \leq 10^3

Input

Input is given from Standard Input in the following format:

N
L_1 L_2 ... L_N

Constraints

Print the number of different triangles that can be formed.

Sample Input 1

4
3 4 2 1

Sample Output 1

1

Only one triangle can be formed: the triangle formed by the first, second, and third sticks.

Sample Input 2

3
1 1000 1

Sample Output 2

0

No triangles can be formed.

Sample Input 3

7
218 786 704 233 645 728 389

Sample Output 3

23

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Diagnostics;
using AtCoder.Collections;
using AtCoder.Mathematics;
using AtCoder.Extensions;

namespace AtCoder
{
    class Program
    {
        static void Main(string[] args)
        {
            var N = ConsoleExtension.ReadInt();
            var array = ConsoleExtension.ReadIntArray().OrderBy(_ => _).ToArray();

            var count = 0;
            for (int i = 0; i < N; i++)
            {
                for (int j = i + 1; j < N; j++)
                {
                    var a = array[i];
                    var b = array[j];

                    count += array.LowerBound(a + b) - (j + 1);
                }
            }

            Console.WriteLine(count);
        }
    }
}

/*
namespace AtCoder.Algorithms
{
    public static partial class Algorithm
    {
    }
}
*/

namespace AtCoder.Collections
{
    /// <summary>
    /// 指定した同じ型のインスタンスの，先頭または末尾への要素の追加，削除が可能な可変サイズのコレクションを表します．
    /// </summary>
    /// <typeparam name="T"><see cref="Deque{T}"/> 内の要素の型を指定します．</typeparam>
    public class Deque<T>
    {
        int dx;
        T[] buf;
        int mask;

        public int Count { get; private set; }
        public Deque() : this(8) { }
        public Deque(int capacity)
        {
            Debug.Assert(capacity == (capacity & -capacity));
            mask = capacity - 1;
            buf = new T[capacity];
        }
        public T this[int index]
        {
            get
            {
                Debug.Assert(0 <= index && index < Count);
                return buf[(dx + index) & mask];
            }
            set
            {
                Debug.Assert(0 <= index && index < Count);
                buf[(dx + index) & mask] = value;
            }
        }
        public void PushFront(T item)
        {
            if (Count == buf.Length) extend();
            dx = (dx + buf.Length - 1) & mask;
            buf[dx] = item;
            Count++;
        }
        public T PopFront()
        {
            Debug.Assert(Count > 0);
            var ret = buf[dx = (dx + 1) & mask];
            Count--;
            return ret;
        }
        public void PushBack(T item)
        {
            if (Count == buf.Length) extend();
            buf[(dx + Count++) & mask] = item;
        }
        public T PopBack()
        {
            Debug.Assert(Count > 0);
            var ret = buf[(dx + --Count) & mask];
            return ret;
        }
        public T Last()
        {
            Debug.Assert(Count > 0);
            return buf[(dx + Count - 1) & mask];
        }
        public T First()
        {
            Debug.Assert(Count > 0);
            return buf[dx];
        }
        void extend()
        {
            var nbuf = new T[buf.Length * 2];
            for (int i = 0; i < buf.Length; i++)
                nbuf[i] = buf[(dx + i) & mask];
            mask = mask * 2 + 1;
            dx = 0;
            buf = nbuf;
        }
        public void Clear() { Count = 0; }
        public bool Any() { return Count != 0; }
        public void Insert(int index, T item)
        {
            Debug.Assert(0 <= index && index <= Count);
            PushFront(item);
            for (int i = 0; i < index; i++)
                this[i] = this[i + 1];
            this[index] = item;
        }
        public T RemoveAt(int index)
        {
            Debug.Assert(0 <= index && index < Count);
            var ret = this[index];
            for (int i = index; i > 0; i--)
                this[i] = this[i - 1];
            PopFront();
            return ret;
        }
        public T[] Items
        {
            get
            {

                var ret = new T[Count];
                for (int i = 0; i < Count; i++)
                    ret[i] = this[i];
                return ret;
            }
        }
    }
    public class PriorityQueue<T> : IEnumerable<T>
    {
        private readonly List<T> _data = new List<T>();
        private readonly IComparer<T> _comparer;
        private readonly bool _isDescending;

        public PriorityQueue(IComparer<T> comparer, bool isDescending = true)
        {
            _comparer = comparer;
            _isDescending = isDescending;
        }

        public PriorityQueue(Comparison<T> comparison, bool isDescending = true)
            : this(Comparer<T>.Create(comparison), isDescending)
        {
        }

        public PriorityQueue(bool isDescending = true)
            : this(Comparer<T>.Default, isDescending)
        {
        }

        public void Enqueue(T item)
        {
            _data.Add(item);
            var childIndex = _data.Count - 1;
            while (childIndex > 0)
            {
                var parentIndex = (childIndex - 1) / 2;
                if (Compare(_data[childIndex], _data[parentIndex]) >= 0)
                    break;
                Swap(childIndex, parentIndex);
                childIndex = parentIndex;
            }
        }

        public T Dequeue()
        {
            var lastIndex = _data.Count - 1;
            var firstItem = _data[0];
            _data[0] = _data[lastIndex];
            _data.RemoveAt(lastIndex--);
            var parentIndex = 0;
            while (true)
            {
                var childIndex = parentIndex * 2 + 1;
                if (childIndex > lastIndex)
                    break;
                var rightChild = childIndex + 1;
                if (rightChild <= lastIndex && Compare(_data[rightChild], _data[childIndex]) < 0)
                    childIndex = rightChild;
                if (Compare(_data[parentIndex], _data[childIndex]) <= 0)
                    break;
                Swap(parentIndex, childIndex);
                parentIndex = childIndex;
            }
            return firstItem;
        }

        public T Peek()
        {
            return _data[0];
        }

        private void Swap(int a, int b)
        {
            var tmp = _data[a];
            _data[a] = _data[b];
            _data[b] = tmp;
        }

        private int Compare(T a, T b)
        {
            return _isDescending ? _comparer.Compare(b, a) : _comparer.Compare(a, b);
        }

        public int Count => _data.Count;

        public IEnumerator<T> GetEnumerator()
        {
            return _data.GetEnumerator();
        }

        IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
    }
    public class PriorityQueue<TKey, TValue> : IEnumerable<TValue>
    {
        private readonly List<KeyValuePair<TKey, TValue>> _data = new List<KeyValuePair<TKey, TValue>>();
        private readonly bool _isDescending;
        private readonly Func<TValue, TKey> _keySelector;
        private readonly IComparer<TKey> _keyComparer;

        public PriorityQueue(Func<TValue, TKey> keySelector, bool isDescending = true)
            : this(keySelector, Comparer<TKey>.Default, isDescending)
        {
        }

        public PriorityQueue(Func<TValue, TKey> keySelector, IComparer<TKey> keyComparer, bool isDescending = true)
        {
            _keySelector = keySelector;
            _keyComparer = keyComparer;
            _isDescending = isDescending;
        }

        public void Enqueue(TValue item)
        {
            _data.Add(new KeyValuePair<TKey, TValue>(_keySelector(item), item));
            var childIndex = _data.Count - 1;
            while (childIndex > 0)
            {
                var parentIndex = (childIndex - 1) / 2;
                if (Compare(_data[childIndex].Key, _data[parentIndex].Key) >= 0)
                    break;
                Swap(childIndex, parentIndex);
                childIndex = parentIndex;
            }
        }

        public TValue Dequeue()
        {
            var lastIndex = _data.Count - 1;
            var firstItem = _data[0];
            _data[0] = _data[lastIndex];
            _data.RemoveAt(lastIndex--);
            var parentIndex = 0;
            while (true)
            {
                var childIndex = parentIndex * 2 + 1;
                if (childIndex > lastIndex)
                    break;
                var rightChild = childIndex + 1;
                if (rightChild <= lastIndex && Compare(_data[rightChild].Key, _data[childIndex].Key) < 0)
                    childIndex = rightChild;
                if (Compare(_data[parentIndex].Key, _data[childIndex].Key) <= 0)
                    break;
                Swap(parentIndex, childIndex);
                parentIndex = childIndex;
            }
            return firstItem.Value;
        }

        public TValue Peek()
        {
            return _data[0].Value;
        }

        private void Swap(int a, int b)
        {
            var tmp = _data[a];
            _data[a] = _data[b];
            _data[b] = tmp;
        }

        private int Compare(TKey a, TKey b)
        {
            return _isDescending ? _keyComparer.Compare(b, a) : _keyComparer.Compare(a, b);
        }

        public int Count => _data.Count;

        public IEnumerator<TValue> GetEnumerator()
        {
            return _data.Select(r => r.Value).GetEnumerator();
        }

        IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
    }
}

namespace AtCoder.Mathematics
{
    public class MathEx
    {
        /// <summary>
        /// 最小公倍数
        /// </summary>
        /// <param name="a"></param>
        /// <param name="b"></param>
        /// <returns></returns>
        static long Lcm(long a, long b)
        {
            return a / Gcd(a, b) * b;
        }
        /// <summary>
        /// 最大公約数
        /// </summary>
        /// <param name="m"></param>
        /// <param name="n"></param>
        /// <returns></returns>
        static long Gcd(long m, long n)
        {
            if (n == 0) return m;
            return Gcd(n, m % n);
        }
    }
    public struct ModInt
    {
        public const long Mod = (int)1e9 + 7;
        public long num;
        public ModInt(long n) { num = n; }
        public override string ToString() { return num.ToString(); }
        public static ModInt operator +(ModInt l, ModInt r) { l.num += r.num; if (l.num >= Mod) l.num -= Mod; return l; }
        public static ModInt operator -(ModInt l, ModInt r) { l.num -= r.num; if (l.num < 0) l.num += Mod; return l; }
        public static ModInt operator *(ModInt l, ModInt r) { return new ModInt(l.num * r.num % Mod); }
        public static ModInt operator /(ModInt l, ModInt r) { return l * Pow(r, Mod - 2); }
        public static implicit operator ModInt(long n) { n %= Mod; if (n < 0) n += Mod; return new ModInt(n); }
        public static ModInt Pow(ModInt v, long k) { return Pow(v.num, k); }
        public static ModInt Pow(long v, long k)
        {
            long ret = 1;
            for (k %= Mod - 1; k > 0; k >>= 1, v = v * v % Mod)
                if ((k & 1) == 1) ret = ret * v % Mod;
            return new ModInt(ret);
        }
        public static ModInt Combination(long n, long r)
        {
            ModInt ans = 1;
            for (int i = 0; i < r; i++)
            {
                ans *= n - i;
            }
            for (int i = 1; i <= r; i++)
            {
                ans /= i;
            }
            return ans;
        }
        public static ModInt Inverse(ModInt v) { return Pow(v, Mod - 2); }
    }
}

namespace AtCoder.Extensions
{
    public static class Extension
    {
        static int binarySearch<T>(IList<T> a, T v, IComparer<T> cmp, bool islb)
        {
            var l = 0;
            var r = a.Count - 1;
            while (l <= r)
            {
                var m = (l + r) / 2;
                var res = cmp.Compare(a[m], v);
                if (res < 0 || (res == 0 && !islb)) l = m + 1;
                else r = m - 1;
            }
            return l;
        }
        /// <summary>
        /// 与えられた比較関数に従って，<paramref name="a"/> の要素のうち，<paramref name="v"/> 以上の要素であるような最小のインデックスを取得します．
        /// </summary>
        public static int LowerBound<T>(this IList<T> a, T v, Comparison<T> f) { return binarySearch(a, v, Comparer<T>.Create(f), true); }
        /// <summary>
        ///　デフォルトの比較関数に従って，<paramref name="a"/> の要素のうち，<paramref name="v"/> 以上の要素であるような最小のインデックスを取得します．
        /// </summary>
        public static int LowerBound<T>(this IList<T> a, T v) { return binarySearch(a, v, Comparer<T>.Default, true); }
        /// <summary>
        /// 与えられた比較関数に従って，<paramref name="a"/> の要素のうち，<paramref name="v"/> 以上の要素であるような最小のインデックスを取得します．
        /// </summary>
        public static int UpperBound<T>(this IList<T> a, T v, Comparison<T> cmp) { return binarySearch(a, v, Comparer<T>.Create(cmp), false); }
        /// <summary>
        ///　デフォルトの比較関数に従って，<paramref name="a"/> の要素のうち，<paramref name="v"/> より真に大きい要素が現れる最小のインデックスを取得します．
        /// </summary>
        public static int UpperBound<T>(this IList<T> a, T v) { return binarySearch(a, v, Comparer<T>.Default, false); }
        public static PriorityQueue<T> AsPriorityQueue<T>(this IEnumerable<T> source, bool isDescending = true)
        {
            var queue = new PriorityQueue<T>(isDescending);
            foreach (var item in source)
            {
                queue.Enqueue(item);
            }

            return queue;
        }
        public static PriorityQueue<TKey, TSource> AsPriorityQueue<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, bool isDescending = true)
        {
            var queue = new PriorityQueue<TKey, TSource>(keySelector, isDescending);
            foreach (var item in source)
            {
                queue.Enqueue(item);
            }

            return queue;
        }
    }
    public static class ConsoleExtension
    {
        public static int ReadInt() => int.Parse(Console.ReadLine());
        public static long ReadLong() => long.Parse(Console.ReadLine());
        public static int[] ReadIntArray() => Console.ReadLine().Split().Select(int.Parse).ToArray();
        public static int[] ReadIntArray(int n) => Enumerable.Range(0, n).Select(_ => ReadInt()).ToArray();
        public static long[] ReadLongArray() => Console.ReadLine().Split(' ').Select(long.Parse).ToArray();
        public static long[] ReadLongArray(int n) => Enumerable.Range(0, n).Select(_ => ReadLong()).ToArray();
    }
}