[leetcode]二叉树的前序遍历

题目描述

给定一个二叉树，返回它的前序遍历。
示例:

输入: [1,null,2,3]  
   1
    \
     2
    /
   3 

输出: [1,2,3]

算法实现

前序遍历首先访问根节点，然后遍历左子树，最后遍历右子树

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 $result = [];
    /**
     * @param TreeNode $root
     * @return Integer[]
     */
    function preorderTraversal($root) {
        $this->doTraverse($root);
        return $this->result;
    }
    function doTraverse($node)
         if($node==null){
             return;
         }
        array_push($this->result,$node->val);
        if($node->left!=null) $this->doTraverse($node->left);
        if($node->right!=null) $this->doTraverse($node->right);
    }
}

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 $result = [];
    public $stack = [];
    /**
     * @param TreeNode $root
     * @return Integer[]
     */
    function preorderTraversal($root) {
        if($root==null){
            return [];
        }
        $this->stack[] = $root;
        do{
            
            $node = array_pop($this->stack);
            $this->result[] = $node->val;
            
            if($node->right!=null){
                $this->stack[] = $node->right;
            }
            if($node->left!=null){
                $this->stack[] = $node->left;
            }
            
        }while(!empty($this->stack));
        
        return $this->result;
    }
}

参考

二叉树的前序遍历