Binary Tree Postorder Traversal LeetCode solution in JavaScript

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class Pair {
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  constructor(node, state) {
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    this.node = node;
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    this.state = state;
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  }
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}
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var postorderTraversal = function (root) {
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  let ans = [];
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  let st = []; // stack
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  root != null && st.push(new Pair(root, 1));
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  while (st.length > 0) {
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    let top = st[st.length - 1];
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    if (top.state == 1) {
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      top.state++;
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      if (top.node.left != null) {
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        st.push(new Pair(top.node.left, 1));
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      }
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    } else if (top.state == 2) {
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      top.state++;
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      if (top.node.right != null) {
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        st.push(new Pair(top.node.right, 1));
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      }
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    } else {
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      ans.push(top.node.val);
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      st.pop();
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    }
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  }
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  return ans;
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};