Solution.ts

/**
 * Definition for a binary tree node.
 * class TreeNode {
 *     val: number
 *     left: TreeNode | null
 *     right: TreeNode | null
 *     constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
 *         this.val = (val===undefined ? 0 : val)
 *         this.left = (left===undefined ? null : left)
 *         this.right = (right===undefined ? null : right)
 *     }
 * }
 */

function postorderTraversal(root: TreeNode | null): number[] {
    if (root == null) return [];
    let stack = [];
    let ans = [];
    let prev = null;
    while (root || stack.length) {
        while (root) {
            stack.push(root);
            root = root.left;
        }
        root = stack.pop();
        if (!root.right || root.right == prev) {
            ans.push(root.val);
            prev = root;
            root = null;
        } else {
            stack.push(root);
            root = root.right;
        }
    }
    return ans;
}