104. 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.

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.

方法1：递归

/**
 * 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;
        if(root->left == NULL && root->right == NULL)
            return 1;     
         return 1 + max(maxDepth(root->left), maxDepth(root->right));
    }
    
};

方法2：非递归

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