[leetcode]二叉树的最大深度

题目描述

给定一个二叉树，找出其最大深度。

二叉树的深度为根节点到最远叶子节点的最长路径上的节点数。

说明: 叶子节点是指没有子节点的节点。

示例：
给定二叉树 [3,9,20,null,null,15,7]，

   3
 / \
9  20
  /  \
 15   7

返回它的最大深度 3 。

算法实现

递归版(php)

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/**
 * Definition for a binary tree node.
 * class TreeNode {
 *     public $val = null;
 *     public $left = null;
 *     public $right = null;
 *     function __construct($value) { $this->val = $value; }
 * }
 */
class Solution {
    public $max_depth=0;

    /**
     * @param TreeNode $root
     * @return Integer
     */
    function maxDepth($root) {
        $this->doTraverse($root,1);
        return $this->max_depth;
    }
    function doTraverse($node,$depth){
        if($node==null){
            return;
        }
        $this->max_depth = max($this->max_depth,$depth);
        ++$depth;
        $this->doTraverse($node->left,$depth);
        $this->doTraverse($node->right,$depth);
    }
}
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/**
 * Definition for a binary tree node.
 * class TreeNode {
 *     public $val = null;
 *     public $left = null;
 *     public $right = null;
 *     function __construct($value) { $this->val = $value; }
 * }
 */
class Solution {

    /**
     * @param TreeNode $root
     * @return Integer
     */
    function maxDepth($root) {
        return $root == null?0:max($this->maxDepth($root->left),$this->maxDepth($root->right))+1;
    }
}

迭代版(php)

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/**
 * Definition for a binary tree node.
 * class TreeNode {
 *     public $val = null;
 *     public $left = null;
 *     public $right = null;
 *     function __construct($value) { $this->val = $value; }
 * }
 */
class Solution {
    public $stack = [];

    /**
     * @param TreeNode $root
     * @return Integer
     */
    function maxDepth($root) {
        $depth = 0;
        if($root==null){
             return $depth;
        }
        $node = $root;
        $this->stack[] = $node;
        do{
            $count = count($this->stack);
            while($count>0){
                $node = array_shift($this->stack);
                if($node->left!=null){
                    $this->stack[] = $node->left;
                };
                if($node->right!=null){
                    $this->stack[] = $node->right;
                }
                $count--;
            }
            $depth++;
        }while(!empty($this->stack));

        return $depth;
    }
}

参考

二叉树的最大深度