145.binary-tree-postorder-traversal.java

/*
 * @lc app=leetcode id=145 lang=java
 *
 * [145] Binary Tree Postorder Traversal
 *
 * https://leetcode.com/problems/binary-tree-postorder-traversal/description/
 *
 * algorithms
 * Hard (46.93%)
 * Total Accepted:    245.5K
 * Total Submissions: 516.7K
 * Testcase Example:  '[1,null,2,3]'
 *
 * 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?
 *
 */
/**
 * 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> list = new ArrayList<>();
    Stack<TreeNode> stack = new Stack<>();
    TreeNode cur = root;
    TreeNode last = null;
    while (cur != null || !stack.empty()) {
      while (cur != null) {
        stack.push(cur);
        cur = cur.left;
      }
      cur = stack.peek();
      if (cur.right == null || cur.right == last) {
        list.add(cur.val);
        stack.pop();
        last = cur;
        cur = null;
      } else {
        cur = cur.right;
      }
    }
    return list;
  }
}