題目來源：145. 二叉樹的後序遍歷

/**
 * 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 postorderTraversal = function(root) {
  if (root === null)
    return [];
  let cur = root;
  let res = [];
  var addPath=(node) =>{
    let arr = [];
    while(node != null){
      arr.push(node.val);
      node = node.right;
    }
    return arr.reverse();
  }
  while (cur) {
    let node 
 
= cur.left;
    if (node !== null) {
      while (node !== null && node.right !== null && node.right !== cur) {
        node = node.right;
      }
      if (node.right === null) {
        node.right = cur;
        cur = cur.left;
        continue;
      } else {
        node.right = null
 
;
        res.push(...addPath(cur.left));
      }
    }
    cur = cur.right;
  }
  res.push(...addPath(root));
  return res;
};

給定一個二叉樹，返回它的後序遍歷。

示例:

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

輸出: [3,2,1]

進階:遞迴演算法很簡單，你可以通過迭代演算法完成嗎？