Longest substring with count of 1s more than 0s

Given a binary string find the longest substring which contains 1’s more than 0’s.

Examples:

Input : 1010
Output : 3
Substring 101 has 1 occuring more number of times than 0.

Input : 101100
Output : 5
Substring 10110 has 1 occuring more number of times than 0.

A simple solution is to one by one consider all the substrings and check if that substring has count of 1 more than 0. If count is more than compare its length with maximum length substring found till now. Time complexity of this solution is O(n^2).

An efficient solution is to use hashing. The idea is to find sum of string traversed until now. Add 1 to the result if current character is ‘1’ else subtract 1. Now the problem reduces to finding largest subarray having sum greater than zero. To find largest subarray having sum greater than zero, we check the value of sum. If sum is greater than zero, then largest subarray with sum greater than zero is arr[0..i]. If sum is less than zero, then find size of subarray arr[j+1..i], where j is index upto which sum of subarray arr[0..j] is sum -1 and j < i and compare that size with largest subarray size found so far. To find index j, store values of sum for arr[0..j] in hash table for all 0 <= j <= i. There might be possibility that a given value of sum repeats. In that case store only first index for which that sum is obtained as it is required to get length of largest subarray and that is obtained from first index occurrence.



Below is the implementation of above approach:

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// CPP program to find largest substring
// having count of 1s more than count
// count of 0s.
#include <bits/stdc++.h>
using namespace std;
  
// Function to find longest substring
// having count of 1s more than count
// of 0s.
int findLongestSub(string bin)
{
    int n = bin.length(), i;
  
    // To store sum.
    int sum = 0;
  
    // To store first occurrence of each
    // sum value.
    unordered_map<int, int> prevSum;
  
    // To store maximum length.
    int maxlen = 0;
  
    // To store current substring length.
    int currlen;
  
    for (i = 0; i < n; i++) {
  
        // Add 1 if current character is 1
        // else subtract 1.
        if (bin[i] == '1')
            sum++;
        else
            sum--;
  
        // If sum is positive, then maximum
        // length substring is bin[0..i]
        if (sum > 0) {
            maxlen = i + 1;
        }
  
        // If sum is negative, then maximum
        // length substring is bin[j+1..i], where
        // sum of substring bin[0..j] is sum-1.
        else if (sum <= 0) {
            if (prevSum.find(sum - 1) != prevSum.end()) {
                currlen = i - prevSum[sum - 1];
                maxlen = max(maxlen, currlen);
            }
        }
  
        // Make entry for this sum value in hash
        // table if this value is not present.
        if (prevSum.find(sum) == prevSum.end())
            prevSum[sum] = i;
    }
  
    return maxlen;
}
  
// Driver code
int main()
{
    string bin = "1010";
    cout << findLongestSub(bin);
    return 0;
}

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Output:

3

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



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Improved By : nik1996