145. Binary Tree Postorder Traversal leetcode typescript

postorderTraversal.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)
 *     }
 * }
 */
//  [1,null,2,3]

function postorderTraversal(root: TreeNode | null): number[] {
    if(!root){
            return [];
        }
    let res:number[]=[]
       

      let stack=[]
        stack.push(root);
        while(stack.length>0){
            let current:TreeNode=stack.pop();
            res.push(current.val);
            if(current.left!=null){
                stack.push(current.left);
            }
            if(current.right!=null){
                stack.push(current.right);
            }
        }
        return res.reverse();
    }