Binary Tree Postorder Traversal

Given a binary tree, return the postorder traversal of its nodes' values.

Example:

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

Output: [3,2,1]

Follow up: Recursive solution is trivial, could you do it iteratively?

function TreeNode(val) {
  this.val = val
  this.left = this.right = null
}

var tree1 = new TreeNode(1)
var tree2 = new TreeNode(2)
var tree3 = new TreeNode(3)
tree2.left = tree3
tree1.left = null
tree1.right = tree2

递归法

/**
 * 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) {
  if (root) {
    return [...postorderTraversal(root.left), ...postorderTraversal(root.right), root.val]
  } else {
    return []
  }
}

颜色标记法(迭代法, 模拟系统栈)

使用颜色标记法剖析树在中序遍历下的递归操作, 思路如下:

1. 将访问过的元素标记为灰色, 未访问过的元素标记为白色;
2. 从栈顶取出访问元素:
   1. 若为灰色元素, 则打印之;
   2. 若为白色元素, 按照中 -> 右 -> 左的顺序推入栈, 同时将白色元素标记为灰色元素;

推荐使用颜色标记法, 它的解题思路适用于解前序、中序、后序遍历。

1
    / \
   2   5
  / \
 3   4

在如上所示树中, 模拟系统栈图解其执行过程如下:

gray  1
white 2
white 5

white 2
white 5

gray  2
white 3
white 4
white 5

/**
 * 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) {
  const printArr = []
  if (!root) return printArr
  const stack = []
  stack.push({ color: 'white', node: root })

  while (stack.length > 0) {
    const { color, node } = stack.pop()
    if (color === 'gray') {
      printArr.push(node.val)
    } else {
      stack.push({ color: 'gray', node })
      node.right && stack.push({ color: 'white', node: node.right })
      node.left && stack.push({ color: 'white', node: node.left })
    }
  }

  return printArr
}

Sister Title

94、144