問題：
給你一個 BST的pre-order / post-order array， reconstruct 出這個BST

pre-order

private int counter_Pre = 0;
public TreeNode restoreBST_Pre(int[] nums) {
    return helper_Pre(nums, Integer.MIN_VALUE, Integer.MAX_VALUE);
}

private TreeNode helper_Pre(int[] nums, int min, int max) {
    if (counter_Pre >= nums.length) {
        return null;
    }
    int val = nums[counter_Pre];
    if (val > min && val < max) {
        counter_Pre++;
        TreeNode root = new TreeNode(val);
        root.left = helper_Pre(nums, min, val);
        root.right = helper_Pre(nums, val, max);
        return root;
    }
    return null;
}

post-order

int counter_Post;
public TreeNode restoreBST_Post(int[] nums) {
    counter_Post = nums.length - 1;
    return helper_Post(nums, Integer.MIN_VALUE, Integer.MAX_VALUE);
}
    
private TreeNode helper_Post(int[] nums, int min, int max) {
    if (counter_Post < 0) {
        return null;
    }
    int val = nums[counter_Post];
    if (min < val && val < max) {
        counter_Post--;
        TreeNode root = new TreeNode(val);
        root.right = helper_Post(nums, val, max);
        root.left = helper_Post(nums, min, val);
        return root;
    }
    return null;
}

Anyway, Good luck, Richardo! -- 09/23/2016