145. 二叉树的后序遍历（算法200题-58）

const helper = (node, list) => {
    if(!node) return
    helper(node.left, list)
    helper(node.right, list)
    list.push(node.val)
}
var postorderTraversal = function (root) {
    const list = []
    helper(root, list)
    return list
};

 */
var postorderTraversal = function (root) {
    const stack = [],
        list = []
    let node = root
    while (node || stack.length){
      while(node){
        stack.push(node);
        list.unshift(node.val);
        node = node.right;
      }
      node = stack.pop();
      node = node.left;
    }
    return list;
};