leetcode刷題-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],
return 3

思路

求二叉樹的最大深度，簡單的分治法，左右子樹的最大深度加一即可

代碼

class Solution {
    public int maxDepth(TreeNode root) {
        // 邊界處理
        if (root == null) {
            return 0;
        }
        // 遞歸出口
        if (root.left == null && root.right == null) {
            return 1;
        }
        // Devide 
        int left = maxDepth(root.left);
        int right = maxDepth(root.right);
        //Merge
        return Math.max(left, right) + 1;
    }
}

traverse 方法

public class Solution {
    private int depth;
    public int maxDepth(TreeNode root) {
        depth = 0;
        traverse(root, 1);
        return depth;
    }
    
    private void traverse(TreeNode root, int curDepth) {
        if (root == null) {
            return;
        }
        
        if (curDepth > depth) {
            depth = curDepth;
        }
        
        traverse(root.left, curDepth + 1);
        traverse(root.right, curDepth + 1);
    }
}