Question:

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.

Thinking:

  • Method:
    • 递归。
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int maxDepth(TreeNode root) {
        return getDepth(root);
    }
    private int getDepth(TreeNode node){
        if(node == null) return 0;
        return Math.max(getDepth(node.left), getDepth(node.right)) + 1;
    }
}

二刷

这道题直接使用递归就行了。

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