Single Num II

Given an array of integers, every element appears three times except
for one. Find that single one.

Note:
Your algorithm should have a linear runtime complexity. Could you
implement it without using extra memory?
3Sum

Given an array S of n integers, are there elements a, b, c in S such
that a + b + c = 0? Find all unique triplets in the array which gives
the sum of zero.

Note:
Elements in a triplet (a,b,c) must be in non-descending order. (ie, a ≤
b ≤ c)
The solution set must not contain duplicate triplets.
    For example, given array S = {-1 0 1 2 -1 -4},

    A solution set is:
    (-1, 0, 1)
    (-1, -1, 2)
PathSum

Given a binary tree and a sum, determine if the tree has a root-to-leaf
path such that adding up all the values along the path equals the given
sum.

For example:
Given the below binary tree and sum = 22,
              5
             / \
            4   8
           /   / \
          11  13  4
         /  \      \
        7    2      1
return true, as there exist a root-to-leaf path 5->4->11->2 which sum
is 22.