145.binary-tree-postorder-traversal.java

/*
 * @lc app=leetcode id=145 lang=java
 *
 * [145] Binary Tree Postorder Traversal
 */

// @lc code=start
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if (root == null) {
            return res;
        }
        Stack<TreeNode> stk = new Stack<>();
        stk.push(root);
        while (!stk.isEmpty()) {
            TreeNode cur = stk.pop();
            res.add(cur.val);
            if (cur.left != null) {
                stk.push(cur.left);
            }
            if (cur.right!= null) {
                stk.push(cur.right);
            }
        }
        // reverse the res
        Collections.reverse(res);
        return res;
    }
}
// @lc code=end

/* class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        postorder(root, res);
        return res;
    }
    void postorder(TreeNode root, List<Integer> res) {
        if (root == null) {
            return;
        }
        postorder(root.left, res);
        postorder(root.right, res);
        res.add(root.val);
    }
} */