104.maximum-depth-of-binary-tree.cpp

/*
 * @lc app=leetcode id=104 lang=cpp
 *
 * [104] Maximum Depth of Binary Tree
 *
 * https://leetcode.com/problems/maximum-depth-of-binary-tree/description/
 *
 * algorithms
 * Easy (67.63%)
 * Likes:    3634
 * Dislikes: 93
 * Total Accepted:    1.1M
 * Total Submissions: 1.6M
 * Testcase Example:  '[3,9,20,null,null,15,7]'
 *
 * Given the root of a binary tree, return its maximum depth.
 * 
 * A binary tree's maximum depth is the number of nodes along the longest path
 * from the root node down to the farthest leaf node.
 * 
 * 
 * Example 1:
 * 
 * 
 * Input: root = [3,9,20,null,null,15,7]
 * Output: 3
 * 
 * 
 * Example 2:
 * 
 * 
 * Input: root = [1,null,2]
 * Output: 2
 * 
 * 
 * Example 3:
 * 
 * 
 * Input: root = []
 * Output: 0
 * 
 * 
 * Example 4:
 * 
 * 
 * Input: root = [0]
 * Output: 1
 * 
 * 
 * 
 * Constraints:
 * 
 * 
 * The number of nodes in the tree is in the range [0, 10^4].
 * -100 <= Node.val <= 100
 * 
 */

// @lc code=start
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int maxDepth1(TreeNode* root) {
        if (root == nullptr)
            return 0;
        int leftDepth = maxDepth(root->left);
        int rightDepth = maxDepth(root->right);
        int depth = 1 + max(leftDepth, rightDepth);
        return depth;
    }
    int maxDepth(TreeNode* root) {
        if (root == nullptr)
            return 0;
        int depth = 0;
        queue<TreeNode*> que;
        que.push(root);
        while (!que.empty()) {
            int size = que.size();
            for (int i = 0; i < size; ++i) {
                TreeNode* node = que.front();
                que.pop();
                if (node->left)
                    que.push(node->left);
                if (node->right)
                    que.push(node->right);
            }
            if (size)
                ++depth;
        }
        return depth;
    }
};
// @lc code=end