Interview Question for Software Development Engineer at Microsoft:

Just implemented this in javascript on the browser (out of pure laziness).

Here's some explanation: The first element on the "preorder" list will always be the root.
1. Find that element in the inorder list and split it.
2. Pop the element from the pre-order list. There you go, just apply recursion now. The following element on the pre-order list will be the new root of either "leftList" or "rightList" (if leftList is empty). Just recursively apply this algorithm on the new lists.

function postOrder(preOrder, inOrder, result) {
    var root = preOrder[0];
    var splitArray;
    var rootIndexInOrder;
    var leftElements;
    var rightElements;
    preOrder.splice(0,1);

    for(var i = 0; i < inOrder.length; ++i) {

        if(inOrder[i] === root) {
            rootIndexInOrder = i;
            break;
        }
    }

    leftElements = inOrder.slice(0, rootIndexInOrder);
    rightElements = inOrder.slice(rootIndexInOrder+1);

    if(leftElements.length !== 0) {
        postOrder(preOrder, leftElements, result);
    }

    if(rightElements.length !== 0) {
        postOrder(preOrder, rightElements, result);
    }

    result.push(root);

}