Maximum Depth of Binary Tree

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.

Analyse: The same as Minimum Depth of Binary Tree. 

1. Recursion

    Runtime: 8ms.

/**
 * Definition for a binary tree node.
 * 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) return 0;
        
        int leftDepth = maxDepth(root->left);
        int rightDepth = maxDepth(root->right);
        
        if(leftDepth == 0 && rightDepth == 0) return 1;
        if(leftDepth == 0) leftDepth = INT_MIN;
        if(rightDepth == 0) rightDepth = INT_MIN;
        
        return 1 + max(leftDepth, rightDepth);
    }
};

2. Iteration: Compute the number of level. Initialize the level to be 0, after dealing with the current level, then level++.

    Runtime: 8ms.

/**
 * Definition for a binary tree node.
 * 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) return 0;
        
        queue<TreeNode* > qu;
        qu.push(root);
        int level = 0;
        while(!qu.empty()){
            int n = qu.size();
            while(n--){
                TreeNode* current = qu.front();
                qu.pop();
                
                if(current->left) qu.push(current->left);
                if(current->right) qu.push(current->right);
            }
            level++;
        }
        return level;
    }
};