Leetcode(104) Maximum Depth of Binary Tree

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Description

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],

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2
3
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5
  3
 / \
9  20
  /  \
 15   7

return its depth = 3.

解法

我是按照按层遍历的方法解的这道题，就是利用队列size确定每层的个数，然后确定总的层数

具体代码如下：

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class Solution {
    public int maxDepth(TreeNode root) {
        int level = 0;
        if (root == null)
            return level;
        Queue<TreeNode> q = new LinkedList<TreeNode>();
        q.offer(root);
        while (!q.isEmpty()) {
            int size = q.size();
            for (int i = 0; i < size; i++) {
                TreeNode tmp = q.poll();
                if (tmp.left != null) {
                    q.offer(tmp.left);
                }
                if (tmp.right != null) {
                    q.offer(tmp.right);
                }
            }
            level++;
        }
        return level;
    }
}

网上看到可以用递归解这题，确实，代码相当简洁

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