Find closest smaller value for every element in array - GeeksforGeeks

04 Feb, 2022
Given an array of integers, find the closest smaller element for every element. If there is no smaller element then print -1
Examples: 
Input : arr[] = {10, 5, 11, 6, 20, 12} Output : 6, -1, 10, 5, 12, 11 
Input : arr[] = {10, 5, 11, 10, 20, 12} Output : 5 -1 10 5 12 11   
A simple solution is to run two nested loops. We pick an outer element one by one. For every picked element, we traverse remaining array and find closest smaller element. Time complexity of this solution is O(n*n)
A better solution is to use sorting. We sort all elements, then for every element, traverse toward left until we find a smaller element (Note that there can be multiple occurrences of an element).
An efficient solution is to use Self Balancing BST (Implemented as set in C++ and TreeSet in Java). In a Self Balancing BST, we can do both insert and closest smaller operations in O(Log n) time.
C++
Java
Python3
C#
Javascript
// C++ program to find closest smaller value for// every array element#include <bits/stdc++.h>using namespace std; void closestSmaller(int arr[], int n){    // Insert all array elements into a TreeSet    set<int> ts;    for (int i = 0; i < n; i++)        ts.insert(arr[i]);     // Find largest smaller element for every    // array element    for (int i = 0; i < n; i++)    {        auto smaller = ts.lower_bound(arr[i]);        if (smaller == ts.begin())            cout << -1 << " ";        else            cout << *(--smaller) << " ";    }} // Driver Codeint main(){    int arr[] = {10, 5, 11, 6, 20, 12};    int n = sizeof(arr) / sizeof(arr[0]);     closestSmaller(arr, n);     return 0;} // This code is contributed by// sanjeev2552
// Java program to find closest smaller value for// every array elementimport java.util.*; class TreeSetDemo {    public static void closestSmaller(int[] arr)    {        // Insert all array elements into a TreeSet        TreeSet<Integer> ts = new TreeSet<Integer>();        for (int i = 0; i < arr.length; i++)            ts.add(arr[i]);         // Find largest smaller element for every        // array element        for (int i = 0; i < arr.length; i++) {            Integer smaller = ts.lower(arr[i]);            if (smaller == null)                System.out.print(-1 + " ");            else                System.out.print(smaller + " ");        }    }     public static void main(String[] args)    {        int[] arr = { 10, 5, 11, 6, 20, 12 };        closestSmaller(arr);    }}
# Python3 program to find closest smaller value# for every array elementimport bisect def closestSmaller(arr, n):         # Insert all array elements into a TreeSet    ts = set()         for i in range(n):        ts.add(arr[i])     # Find largest smaller element for every    # array element    for i in range(n):        smaller = bisect.bisect_left(list(ts), arr[i])                 if (smaller == 0):            print(-1, end = " ")        else:            print((list(ts)[smaller - 1]), end = " ")            smaller -= 1 # Driver Codearr = [ 10, 5, 11, 6, 20, 12 ]n = len(arr) closestSmaller(arr, n) # This code is contributed by rohitsingh07052
// C# program to find closest smaller value for// every array elementusing System;using System.Linq;using System.Collections.Generic; public class TreeSetDemo {  public static void closestSmaller(int[] arr)  {    // Insert all array elements into a TreeSet    SortedSet<int> ts = new SortedSet<int>();    for (int i = 0; i < arr.Length; i++)      ts.Add(arr[i]);     // Find largest smaller element for every    // array element    for (int i = 0; i < arr.Length; i++) {      int smaller = lower_bound(ts, arr[i]);       if (smaller == 0)        Console.Write(-1 + " ");      else        Console.Write(smaller + " ");    }  }  public static int lower_bound(SortedSet<int> s, int val)  {    List<int> temp = new List<int>();    temp.AddRange(s);    temp.Sort();    temp.Reverse();     if (temp.IndexOf(val) + 1 == temp.Count)      return -1;    return temp[temp.IndexOf(val) + 1];  }   public static void Main(String[] args)  {    int[] arr = { 10, 5, 11, 6, 20, 12 };    closestSmaller(arr);  }} // This code is contributed by Rajput-Ji
<script>// javascript program to find closest smaller value for// every array elementclass TreeSetDemo {    function closestSmaller(arr)    {             // Insert all array elements into a TreeSet        var ts = new Set();        for (i = 0; i < arr.length; i++)            ts.add(arr[i]);         // Find largest smaller element for every        // array element        for (i = 0; i < arr.length; i++) {            var smaller = upper_bound(ts, arr[i]);            if (smaller == null)                document.write(-1 + " ");            else                document.write(smaller + " ");        }    }    function upper_bound(s, val)    {        let temp = [...s];        temp.sort((a, b) => b - a);        return temp[temp.indexOf(val) + 1];    }          var arr = [ 10, 5, 11, 6, 20, 12 ];        closestSmaller(arr);         // This code is contributed by Rajput-Ji</script>
6 -1 10 5 12 11
 
Time Complexity : O(n Log n) 
sanjeev2552
rohitsingh07052
Rajput-Ji
java-treeset
Self-Balancing-BST
Arrays
Binary Search Tree
Java Programs
Arrays
Binary Search Tree
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