字节跳动 144. Binary Tree Preorder T

https://leetcode.com/problems/binary-tree-preorder-traversal/

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
var preorderTraversal = function(root) {
    let s = [];
    let p = root;
    let res = [];
    while(p!=null || s.length!=0){
        while(p!=null){
            res.push(p.val);
            s.push(p);
            p = p.left;
        }
        if(s.length!=0){
            p = s.pop();
            p = p.right;
        }
    }
    return res;
};

非递归方法重点要掌握先一路往左走（外while和内while），然后一边走一边入栈。接下来一路从底部往上走，如果能往上走（外while内if）就将指针切到右子节点去测试是否能更深一层走。