[LeetCode] 145. Binary Tree Postorder Traversal

题：https://leetcode.com/problems/binary-tree-postorder-traversal/description/

题目

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?

题目大意

后续遍历

思路

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        TreeNode lastp = null;
        while(root!= null || !stack.isEmpty()){
            while(root != null){
                stack.push(root);
                root = root.left;
            }
            if(!stack.isEmpty()){
                TreeNode top = stack.peek();
                if(top.right != null && lastp!=top.right){
                    root = top.right;
                }
                else{
                    res.add(top.val);
                    lastp = top;
                    stack.pop();   
                }
            }
        }
        return res;
    }
}