LeetCode104. Maximum Depth of Binary Tree 二叉樹的最大深度

1. problem description

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.

Note: A leaf is a node with no children.

example:
Given binary tree [3,9,20,null,null,15,7],
  3
 /  \
9   20
   /   \
 15    7
return its depth = 3.

2. solution

/**
 * 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 == NULL)
        return 0;
    
	    int res = 0;
	    queue<TreeNode *> q;
	    q.push(root);
	    while(!q.empty())
    	{
	        ++ res;
	        for(int i = 0, n = q.size(); i < n; ++ i)
	        {
	            TreeNode *p = q.front();
	            q.pop();
	            
	            if(p -> left != NULL)
	                q.push(p -> left);
	            if(p -> right != NULL)
	                q.push(p -> right);
	        }
    	}
    
    	return res;
    }
};