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Given an integer. Find how many **structurally unique binary search trees **are there that stores the values from 1 to that integer (inclusive).

**Example 1:**

Input:N = 2Output:2Explanation:for N = 2, there are 2 unique BSTs 1 2 \ / 2 1

**Example 2:**

Input:N = 3Output:5Explanation:for N = 3, there are 5 possible BSTs 1 3 3 2 1 \ / / / \ \ 3 2 1 1 3 2 / / \ \ 2 1 2 3

**Your Task:**

You don't need to read input or print anything. Your task is to complete the function **numTrees()** which takes the integer N as input and returns the total number of Binary Search Trees possible with keys [1.....N] inclusive. Since the answer can be very large, return the **answer modulo 10e9 + 7**.

**Expected Time Complexity:** O(N^{2}).

**Expected Auxiliary Space:** O(N).

**Constraints:**

1<=N<=1000

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