[leetcode]144. Binary Tree Preorder Traversal -- JavaScript 代码

/** * Definition for a binary tree node. * function TreeNode(val) { *     this.val = val; *     this.left = this.right = null; * } *//** * @param {TreeNode} root * @return {number[]} */var preorderTraversal = function(root) {    var ret = [];    if(root === null){        return ret;    }    var stack = [];    var cur = root;    while(cur!==null){        ret.push(cur.val);        left = cur.left;        right  = cur.right;        if(left !== null && right !== null){            stack.push(right);            cur = left;        }else if(left === null && right === null){            if(stack.length>0){                cur = stack.pop();            }else{                cur = null;            }        }else if(left === null){            cur = right;        }else{            cur = left;        }        //console.log(cur);    }    return ret;};

这道题用递归可以很好的解决，可是题目说是否可以不用递归。

于是就只好利用栈来完成了。

思路也很清晰。