104. Maximum Depth of Binary Tre

一：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;
        }
        
        int right = maxDepth(root->right);
        int left = maxDepth(root->left);
        
        return max(right, left) + 1;
    }
};

二：广度优先遍历

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