【leetcode】Binary Tree Postorder Traversal

题目：

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

For example:
Given binary tree {1,#,2,3},

   1    \     2    /   3

return [3,2,1].

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

如下列出三种解法：一种迭代的和两种iterative的

import java.util.ArrayList;import java.util.List;import java.util.Stack;public class Binary_Tree_Postorder_Traversal {public static class TreeNode {       int val;       TreeNode left;       TreeNode right;   TreeNode(int x) { val = x; }}public static void main(String[] args) {        TreeNode r1 = new TreeNode(1);          TreeNode r2 = new TreeNode(2);          TreeNode r3 = new TreeNode(3);          TreeNode r4 = new TreeNode(4);          TreeNode r5 = new TreeNode(5);        TreeNode r6 = new TreeNode(6);                  r1.left = r2;          r1.right = r3;          r2.left = r4;          r2.right = r5;          r3.right = r6;         List pr = postorderTraversal1(r1);                int size = pr.size();                System.out.println("the size is " + size);                for (int i =0 ; i< size; i++) {        System.out.println(pr.get(i));        }}public static List<Integer> postorderTraversal1(TreeNode root) {ArrayList<Integer> result = new ArrayList<Integer>();if (root == null)return result;result.addAll(postorderTraversal1(root.left));result.addAll(postorderTraversal1(root.right));result.add(root.val);return result;}public static List<Integer> postorderTraversal2(TreeNode root) {     ArrayList<Integer> result = new ArrayList<Integer>();    Stack<TreeNode> stack = new Stack<TreeNode>();    Stack<TreeNode> output = new Stack<TreeNode>();        if (root == null) return result;        stack.push(root);        while(!stack.empty()) {    TreeNode cur = stack.pop();    output.push(cur);        if(cur.left != null) {    stack.push(cur.left);    }    if(cur.right != null) {        stack.push(cur.right);    }    }        while(!output.isEmpty()) {    result.add(output.pop().val);    }    return result;}public List<Integer> postorderTraversal3(TreeNode root) {    ArrayList<Integer> result = new ArrayList<Integer>();    Stack<TreeNode> stack = new Stack<TreeNode>();    TreeNode prev = null; // previously traversed node    TreeNode curr = root;    if (root == null) {        return result;    }    stack.push(root);    while (!stack.empty()) {        curr = stack.peek();        if (prev == null || prev.left == curr || prev.right == curr) { // traverse down the tree            if (curr.left != null) {                stack.push(curr.left);            } else if (curr.right != null) {                stack.push(curr.right);            }        } else if (curr.left == prev) { // traverse up the tree from the left            if (curr.right != null) {                stack.push(curr.right);            }        } else { // traverse up the tree from the right            result.add(curr.val);            stack.pop();        }        prev = curr;    }    return result;}}