0145 - Binary Tree Postorder Traversal

0145 - Binary Tree Postorder Traversal

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

Examples

Input: root = [1,null,2,3] Output: [3,2,1]

Input: root = [] Output: []

Input: root = [1] Output: [1]

Input: root = [1,2] Output: [2,1] Input: root = [1,null,2] Output: [2,1]

Constraints

The number of nodes in the tree is in the range [0, 100]. -100 <= Node.val <= 100

Java Solution

/**
 * 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) {
    LinkedList<Integer> ans = new LinkedList<>();
    Stack<TreeNode> stack = new Stack<>();
    
    if (root == null) return ans;

    stack.push(root);
    while (!stack.isEmpty()) {
        TreeNode cur = stack.pop();
        ans.addFirst(cur.val);
        if (cur.left != null) {
            stack.push(cur.left);
        }
        if (cur.right != null) {
            stack.push(cur.right);
        } 
    }
    return ans;
    }
}