Unique Binary Search Trees @LeetCode

PROBLEM :

Given n, how many structurally unique BST's (binary search trees) that store values 1...n?

For example,
Given n = 3, there are a total of 5 unique BST's.

   1         3     3      2      1
    \       /     /      / \      \
     3     2     1      1   3      2
    /     /       \                 \
   2     1         2                 3

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SIMPLE c++ IMPLEMENTATION :( Recursive Solution - Complexity O(2^n) )
--------------------------------------------------------------------------------

class Solution {
public:
    int numTrees(int n) {
        if(n==1||n==0)
            return 1 ;
           
        int leftTrees=0, rightTrees=0 ,Total=0 ;
       
        for(int i=1;i<=n;i++){
            leftTrees=numTrees(i-1) ;
            rightTrees=numTrees(n-i) ;
            Total=Total+(leftTrees*rightTrees) ;
        }
        return Total ;
    }
};

--------------------------------------------------------------------------------
SIMPLE c++ IMPLEMENTATION :( Iterative Solution - Complexity O(n^2))
--------------------------------------------------------------------------------

class Solution {
public:
    int numTrees(int n) {
        if(n==0||n==1)
            return 1 ;
        if(n==2)
            return 2 ;
           
        int arr[n+1] ;
       
        arr[0]=1 ;
        arr[1]=1 ;
        arr[2]=2 ;
       
        for(int i=3;i<=n;i++){
            int sum=0 ;
            for(int j=0;j<i;j++){
                sum+=arr[j]*arr[i-j-1] ;
            }
            arr[i]=sum ;
        }
        return arr[n] ;
    }
};

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