104 Maximum Depth of Binary Tree

LeetCode

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, 104]. -100 <= Node.val <= 100

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前序遍历，每向下一层深度+1，向上一层深度-1，取最大深度

class Solution {
    int max=0,cur=0;
    public int maxDepth(TreeNode root) {
        if(root==null) return 0;
        preOrder(root);
        return max;
    }
    
    private void preOrder(TreeNode t){
        ++cur;
        max=Math.max(max,cur);
        if(t.left!=null){
            preOrder(t.left);
        }
        if(t.right!=null){
            preOrder(t.right);
        }
        --cur;
    }
}

其实还有更简洁的写法，每个节点下面的最大深度，是左右子树的最大深度

class Solution {
    public int maxDepth(TreeNode root) {
        if(root==null) return 0;
        return Math.max(maxDepth(root.left),maxDepth(root.right))+1;
    }
}