二叉树的最大深度 Maximum Depth of Binary Tree

题目

给定一个二叉树，找出其最大深度

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],

3

/ \ 9 20 / \ 15 7 return its depth = 3.

思路&题解

递归求解代码

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public int maxDepth(TreeNode root) {
    if(root==null){
        return 0;
    } else {
        return Math.max(maxDepth(root.left), maxDepth(root.right)) +1;
    }

}

https://leetcode-cn.com/problems/maximum-depth-of-binary-tree/solution/

import javafx.util.Pair;
import java.lang.Math;

class Solution {
  public int maxDepth(TreeNode root) {
    Queue<Pair<TreeNode, Integer>> stack = new LinkedList<>();
    if (root != null) {
      stack.add(new Pair(root, 1));
    }

    int depth = 0;
    while (!stack.isEmpty()) {
      Pair<TreeNode, Integer> current = stack.poll();
      root = current.getKey();
      int current_depth = current.getValue();
      if (root != null) {
        depth = Math.max(depth, current_depth);
        stack.add(new Pair(root.left, current_depth + 1));
        stack.add(new Pair(root.right, current_depth + 1));
      }
    }
    return depth;
  }
};