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

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

class Solution {
public:
    int singleNumber(int A[], int n) {
        int result  = 0;
        for(int i = 0;i < n;i++)
            result ^= A[i];
        return result;
    }
};


2 

Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int maxDepth(TreeNode *root) {
        if(root==NULL)
            return 0;
        int leftDep,rightDep;
        leftDep=rightDep=0;
        if(root->left!=NULL)
            leftDep=maxDepth(root->left);
        if(root->right!=NULL)
            rightDep=maxDepth(root->right);
        return 1+(leftDep>rightDep?leftDep:rightDep);
    }
};

3 Given two binary trees, write a function to check if they are equal or not.

Two binary trees are considered equal if they are structurally identical and the nodes have the same value.



/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isSameTree(TreeNode *p, TreeNode *q) {
        if(p==NULL&&q==NULL)
            return true;
        if((p==NULL&&q!=NULL)||(q==NULL&&p!=NULL))
            return false;
        if(p->val!=q->val)
            return false;
        return isSameTree(p->left,q->left)&&isSameTree(p->right,q->right);
    }
};