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Position of Elements which are equal to sum of all Preceding elements

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  • Last Updated : 10 Mar, 2022
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Given an array Arr[] of N of positive integers. The task is to find positions of all the elements which are equal to the sum of all preceding elements. If no such element exists print -1.
Examples: 
 

Input : Arr[] = {1, 2, 3, 6, 3, 15, 5} 
Output :3 4 6
Here, the element at index “3” i.e. 3 is equal to the sum of preceding elements (1 + 2). 
Similarly, at index 4, 6 = 1+2+3 (sum of all preceding elements). 
And element at index 6 i.e. 15 = 1 + 2 + 3 + 6 + 3.
Input: Arr[] = {7, 5, 17, 25} 
Output: -1 
 

 

Approach: 
While traversing the array Arr[], maintain a sum variable that store the sum of elements till i – 1. Compare the sum with current element Arr[i]. If it is equal, push the index of this element into the answer vector.
Below is the implementation of the above approach: 
 

C++




// C++ implementation
#include <bits/stdc++.h>
using namespace std;
 
// function to return valid indexes
vector<int> find_idx(int ar[], int n)
{
 
    // Vector to store the answer
    vector<int> answer;
 
    // Initial sum would always
    // be first element
    int sum = ar[0];
 
    for (int i = 1; i < n; i++) {
 
        // Check if sum till now
        // is equal to current element
        if (sum == ar[i]) {
            answer.push_back(i + 1);
        }
 
        // Updating the sum by
        // adding the current
        // element in each
        // iteration.
        sum += ar[i];
    }
 
    return answer;
}
 
// Driver code
int main()
{
    int ar[] = { 1, 2, 3, 6, 3, 15, 5 };
    int n = sizeof(ar) / sizeof(int);
 
    vector<int> ans = find_idx(ar, n);
 
    if (ans.size() != 0) {
        for (int i : ans) {
            cout << i << " ";
        }
    }
    else {
        cout << "-1";
    }
 
    cout << endl;
 
    return 0;
}

Java




// Java implementation of the approach
import java.util.*;
 
class GFG
{
     
// function to return valid indexes
static Vector<Integer> find_idx(int ar[], int n)
{
 
    // Vector to store the answer
    Vector<Integer> answer = new Vector<Integer>();
 
    // Initial sum would always
    // be first element
    int sum = ar[0];
 
    for (int i = 1; i < n; i++)
    {
 
        // Check if sum till now
        // is equal to current element
        if (sum == ar[i])
        {
            answer.add(i + 1);
        }
 
        // Updating the sum by adding the
        // current element in each iteration.
        sum += ar[i];
    }
    return answer;
}
 
// Driver code
public static void main(String[] args)
{
    int ar[] = { 1, 2, 3, 6, 3, 15, 5 };
    int n = ar.length;
 
    Vector<Integer> ans = find_idx(ar, n);
 
    if (ans.size() != 0)
    {
        for (int i : ans)
        {
            System.out.print(i + " ");
        }
    }
    else
    {
        System.out.println("-1");
    }
}
}
 
// This code is contributed by 29AjayKumar

Python3




# Python3 implementation of the above approach
 
# function to return valid indexes
def find_idx(ar, n) :
 
    # Vector to store the answer
    answer = [];
 
    # Initial sum would always
    # be first element
    sum = ar[0];
 
    for i in range(1, n) :
 
        # Check if sum till now
        # is equal to current element
        if (sum == ar[i]) :
            answer.append(i + 1);
 
        # Updating the sum by
        # adding the current
        # element in each
        # iteration.
        sum += ar[i];
 
    return answer;
 
# Driver code
if __name__ == "__main__" :
 
    ar = [ 1, 2, 3, 6, 3, 15, 5 ];
    n = len(ar);
 
    ans = find_idx(ar, n);
 
    if (len(ans) != 0) :
         
        for i in ans :
            print(i, end = " ");
             
    else :
         
        print("-1");
 
    print();
 
# This code is contributed by AnkitRai01

C#




// C# implementation of the approach
using System;
using System.Collections.Generic;
     
class GFG
{
     
// function to return valid indexes
static List<int> find_idx(int []ar, int n)
{
 
    // Vector to store the answer
    List<int> answer = new List<int>();
 
    // Initial sum would always
    // be first element
    int sum = ar[0];
 
    for (int i = 1; i < n; i++)
    {
 
        // Check if sum till now
        // is equal to current element
        if (sum == ar[i])
        {
            answer.Add(i + 1);
        }
 
        // Updating the sum by adding the
        // current element in each iteration.
        sum += ar[i];
    }
    return answer;
}
 
// Driver code
public static void Main(String[] args)
{
    int []ar = { 1, 2, 3, 6, 3, 15, 5 };
    int n = ar.Length;
 
    List<int> ans = find_idx(ar, n);
 
    if (ans.Count != 0)
    {
        foreach (int i in ans)
        {
            Console.Write(i + " ");
        }
    }
    else
    {
        Console.WriteLine("-1");
    }
}
}
 
// This code is contributed by Princi Singh

Javascript




<script>
// Javascript implementation
 
// function to return valid indexes
function find_idx(ar, n) {
 
    // Vector to store the answer
    let answer = [];
 
    // Initial sum would always
    // be first element
    let sum = ar[0];
 
    for (let i = 1; i < n; i++) {
 
        // Check if sum till now
        // is equal to current element
        if (sum == ar[i]) {
            answer.push(i + 1);
        }
 
        // Updating the sum by
        // adding the current
        // element in each
        // iteration.
        sum += ar[i];
    }
 
    return answer;
}
 
// Driver code
 
let ar = [1, 2, 3, 6, 3, 15, 5];
let n = ar.length;
 
let ans = find_idx(ar, n);
 
if (ans.length != 0) {
    for (let i of ans) {
        document.write(i + " ");
    }
}
else {
    document.write("-1");
}
 
document.write("<br>");
 
 
// This code is contributed by gfgking.
</script>

Output: 

3 4 6

 

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


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