【Leetcode】【Easy】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.

递归解题思路：

①结点为空时，返回0；②当前结点的深度 = max（两个孩子结点各自最大深度）+ 1

/**
 * 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) 
            return 0;
        
        return max(maxDepth(root->left), 
                maxDepth(root->right)) + 1;
    }
};

迭代的解法：

用queue先进先出的特性，按层遍历，最后返回遍历的层数。

class Solution {
public:
    int maxDepth(TreeNode *root) {
        if (!root) {
            return 0;
        }
        
        queue<TreeNode *> nodeQue;
        nodeQue.push(root);
        int levelNodesCnt;
        int depth = 0;
        
        while (!nodeQue.empty()) {
            depth++;
            levelNodesCnt = nodeQue.size();
            while (levelNodesCnt--) {
                if (nodeQue.front()->left)
                    nodeQue.push(nodeQue.front()->left);
                if (nodeQue.front()->right)
                    nodeQue.push(nodeQue.front()->right);
                nodeQue.pop();
            }
        }
        
        return depth;
    }
};

附录：

C++ queue使用