145.二叉树的后序遍历.js

/*
 * @lc app=leetcode.cn id=145 lang=javascript
 *
 * [145] 二叉树的后序遍历
 */

const { traverse } = require('@babel/core')

// @lc code=start
/**
 * 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[]}
 */
// solution1
// var postorderTraversal = function(root) {
//   let res = []
//   if (!root) return res

//   res = res.concat(postorderTraversal(root.left))
//   res = res.concat(postorderTraversal(root.right))
//   res.push(root.val)
//   return res
// }

// solution2
// var postorderTraversal = function(root) {
//   let res = []
//   traverse(root)
//   return res

//   function traverse(node) {
//     if (!node) return
//     traverse(node.left)
//     traverse(node.right)
//     res.push(node.val)
//   }
// }

// solution3: iterative
var postorderTraversal = function(root) {
  const res = []
  if (root == null) {
    return res
  }
  const stack = [root]
  while (stack.length) {
    let cur = stack.pop()
    res.push(cur.val)
    cur.left && stack.push(cur.left)
    cur.right && stack.push(cur.right)
  }
  return res.reverse() // remember to reverse
}

// @lc code=end

module.exports = { postorderTraversal }