Equilibrium Point

Problem Statement - link #

Given an array A of n positive numbers. The task is to find the first Equilibium Point in the array. Equilibrium Point in an array is a position such that the sum of elements before it is equal to the sum of elements after it.

Note: Retun the index of Equilibrium point. (1-based index)

Your Task: The task is to complete the function equilibriumPoint() which takes the array and n as input parameters and returns the point of equilibrium. Return -1 if no such point exists.

Expected Time Complexity: O(n)
Expected Auxiliary Space: O(1)

Examples #

Example 1:

Input: 
n = 5 
A[] = {1,3,5,2,2} 
Output: 3 
Explanation: For second test case 
equilibrium point is at position 3 
as elements before it (1+3) = 
elements after it (2+2). 

Example 2:

Input:
n = 1
A[] = {1}
Output: 1
Explanation:
Since its the only element hence
its the only equilibrium point.

Constraints #

• 1 <= n <= 10^6
• 1 ≤ A[i] <= 10^8

Solutions #

class Solution{
    public:
    // Function to find equilibrium point in the array.
    // a: input array
    // n: size of array
    int equilibriumPoint(long long a[], int n) {
    
        // Your code here
        int rsum = a[0];
        for(int i=1; i<n; i++) rsum+=a[i];
        int lsum = 0; 
        for(int i=0; i<n; i++){
            rsum -= a[i];
            if(lsum == rsum) 
            return i+1;
            lsum += a[i];
        }
        return -1;
    }
};