Given an array of 0s and 1s, find the position of 0 to be replaced with 1 to get longest continuous sequence of 1s. Expected time complexity is O(n) and auxiliary space is O(1).

Example:

Input: arr[] = {1, 1, 0, 0, 1, 0, 1, 1, 1, 0, 1, 1, 1} Output: Index 9 Assuming array index starts from 0, replacing 0 with 1 at index 9 causes the maximum continuous sequence of 1s. Input: arr[] = {1, 1, 1, 1, 0} Output: Index 4

**We strongly recommend to minimize the browser and try this yourself first.**

A **Simple Solution** is to traverse the array, for every 0, count the number of 1s on both sides of it. Keep track of maximum count for any 0. Finally return index of the 0 with maximum number of 1s around it. The time complexity of this solution is O(n^{2}).

Using an **Efficient Solution**, the problem can solved in O(n) time. The idea is to keep track of three indexes, current index (*curr*), previous zero index (*prev_zero*) and previous to previous zero index (*prev_prev_zero*). Traverse the array, if current element is 0, calculate the difference between *curr *and *prev_prev_zero* (This difference minus one is the number of 1s around the prev_zero). If the difference between *curr *and *prev_prev_zero* is more than maximum so far, then update the maximum. Finally return index of the prev_zero with maximum difference.

Following are C++ and Java implementations of the above algorithm.

## C++

// C++ program to find Index of 0 to be replaced with 1 to get // longest continuous sequence of 1s in a binary array #include<iostream> using namespace std; // Returns index of 0 to be replaced with 1 to get longest // continuous sequence of 1s. If there is no 0 in array, then // it returns -1. int maxOnesIndex(bool arr[], int n) { int max_count = 0; // for maximum number of 1 around a zero int max_index; // for storing result int prev_zero = -1; // index of previous zero int prev_prev_zero = -1; // index of previous to previous zero // Traverse the input array for (int curr=0; curr<n; ++curr) { // If current element is 0, then calculate the difference // between curr and prev_prev_zero if (arr[curr] == 0) { // Update result if count of 1s around prev_zero is more if (curr - prev_prev_zero > max_count) { max_count = curr - prev_prev_zero; max_index = prev_zero; } // Update for next iteration prev_prev_zero = prev_zero; prev_zero = curr; } } // Check for the last encountered zero if (n-prev_prev_zero > max_count) max_index = prev_zero; return max_index; } // Driver program int main() { bool arr[] = {1, 1, 0, 0, 1, 0, 1, 1, 1, 0, 1, 1, 1}; int n = sizeof(arr)/sizeof(arr[0]); cout << "Index of 0 to be replaced is " << maxOnesIndex(arr, n); return 0; }

## Java

// Java program to find Index of 0 to be replaced with 1 to get // longest continuous sequence of 1s in a binary array import java.io.*; class Binary { // Returns index of 0 to be replaced with 1 to get longest // continuous sequence of 1s. If there is no 0 in array, then // it returns -1. static int maxOnesIndex(int arr[], int n) { int max_count = 0; // for maximum number of 1 around a zero int max_index=0; // for storing result int prev_zero = -1; // index of previous zero int prev_prev_zero = -1; // index of previous to previous zero // Traverse the input array for (int curr=0; curr<n; ++curr) { // If current element is 0, then calculate the difference // between curr and prev_prev_zero if (arr[curr] == 0) { // Update result if count of 1s around prev_zero is more if (curr - prev_prev_zero > max_count) { max_count = curr - prev_prev_zero; max_index = prev_zero; } // Update for next iteration prev_prev_zero = prev_zero; prev_zero = curr; } } // Check for the last encountered zero if (n-prev_prev_zero > max_count) max_index = prev_zero; return max_index; } // Driver program to test above function public static void main(String[] args) { int arr[] = {1, 1, 0, 0, 1, 0, 1, 1, 1, 0, 1, 1, 1}; int n = arr.length; System.out.println("Index of 0 to be replaced is "+ maxOnesIndex(arr, n)); } } /* This code is contributed by Devesh Agrawal */

## Python3

# Python program to find Index # of 0 to be replaced with 1 to get # longest continuous sequence # of 1s in a binary array # Returns index of 0 to be # replaced with 1 to get longest # continuous sequence of 1s. # If there is no 0 in array, then # it returns -1. def maxOnesIndex(arr,n): # for maximum number of 1 around a zero max_count = 0 # for storing result max_index =0 # index of previous zero prev_zero = -1 # index of previous to previous zero prev_prev_zero = -1 # Traverse the input array for curr in range(n): # If current element is 0, # then calculate the difference # between curr and prev_prev_zero if (arr[curr] == 0): # Update result if count of # 1s around prev_zero is more if (curr - prev_prev_zero > max_count): max_count = curr - prev_prev_zero max_index = prev_zero # Update for next iteration prev_prev_zero = prev_zero prev_zero = curr # Check for the last encountered zero if (n-prev_prev_zero > max_count): max_index = prev_zero return max_index # Driver program arr = [1, 1, 0, 0, 1, 0, 1, 1, 1, 0, 1, 1, 1] n = len(arr) print("Index of 0 to be replaced is ", maxOnesIndex(arr, n)) # This code is contributed # by Anant Agarwal.

Output:

Index of 0 to be replaced is 9

Time Complexity: O(n)

Auxiliary Space: O(1)

This article is contributed by **Ankur Singh**. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.