145. 二叉树的后序遍历

题目

给定一个二叉树，返回它的后序遍历。

示例:

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

输出: [3,2,1]

题解

递归

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
var postorderTraversal = function(root) {
  let ans = [];
  const postOrder = node => {
    if (!node) return;
    postOrder(node.left);
    postOrder(node.right);
    ans.push(node.val);
  };
  postOrder(root);
  return ans;
};

迭代

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
var postorderTraversal = function(root) {
  let ans = [];
  if (!root) return ans;
  let s = [root];
  let pre = null;
  while (s.length) {
    const node = s[s.length - 1];
    // 当前节点是叶节点或pre节点是当前节点的子节点
    if (
      (!node.left && !node.right) ||
      (pre && (pre == node.left || pre == node.right))
    ) {
      ans.push(node.val);
      s.pop();
      pre = node;
    } else {
      if (node.right) s.push(node.right);
      if (node.left) s.push(node.left);
    }
  }
  return ans;
};