Count Substrings with equal number of 0s, 1s and 2s

Given a string that consists of only 0s, 1s and 2s, count the number of substrings that have an equal number of 0s, 1s, and 2s.

Examples:

Input: str = “0102010”
Output:  2
Explanation: Substring str[2, 4] = “102” and substring str[4, 6] = “201” has equal number of 0, 1 and 2

Input: str = “102100211”
Output: 5

Brute Force: To solve the problem using this approach follow the below idea:

Iterate through all substrings of str using nested loops and check whether they contain equal 0,1 and 2 or not.

Time Complexity: O(N³)
Auxiliary Space: O(1)

Count Substrings with equal number of 0s, 1s and 2s using Hashing:

Traverse through the string and keep track of counts of 0, 1, and 2 and make a difference pair of (zeroes – ones, zeroes – twos) and increase the answer count if this difference pair is seen before and at every index increase the count of this difference pair in the map

Follow the given steps to solve the problem:

• Declare a map to store the difference pair and three variables to store the count of 0’s, 1’s and 2’s 
• Traverse the string and keep track of the count of 0’s, 1’s, and 2’s
• At each index make a difference pair of (zeroes – ones, zeroes – twos)
• Using the map check if this pair is seen before, if it is so then increase the result count
• Then, increase the count of this pair in the map
• Return the result

Below is the implementation of the above approach:

C++

// C++ program to find substring with equal
// number of 0's, 1's and 2's
#include <bits/stdc++.h>
using namespace std;

// Method to count number of substring which
// has equal 0, 1 and 2
int getSubstringWithEqual012(string str)
{
    int N = str.length();

    // map to store, how many times a difference
    // pair has occurred previously
    map<pair<int, int>, int> mp;
    mp[make_pair(0, 0)] = 1;

    //  zc (Count of zeroes), oc(Count of 1s)
    //  and tc(count of twos)
    //  In starting all counts are zero
    int zc = 0, oc = 0, tc = 0;

    //  looping into string
    int res = 0; // Initialize result
    for (int i = 0; i < N; ++i) {

        // increasing the count of current character
        if (str[i] == '0')
            zc++;
        else if (str[i] == '1')
            oc++;
        else
            tc++; // Assuming that string doesn't contain
                  // other characters

        // making pair of differences (z[i] - o[i],
        // z[i] - t[i])
        pair<int, int> tmp = make_pair(zc - oc, zc - tc);

        // Count of previous occurrences of above pair
        // indicates that the subarrays forming from
        // every previous occurrence to this occurrence
        // is a subarray with equal number of 0's, 1's
        // and 2's
        res = res + mp[tmp];

        // Increasing the count of current difference
        // pair by 1
        mp[tmp]++;
    }

    return res;
}

//  Driver's code
int main()
{
    string str = "0102010";

    // Function call
    cout << getSubstringWithEqual012(str) << endl;
    return 0;
}

Java

// Java program to find substring with equal
// number of 0's, 1's and 2's

import java.io.*;
import java.util.*;

class GFG {

    // Method to count number of substring which
    // has equal 0, 1 and 2
    private static int getSubstringWithEqual012(String str)
    {

        // map to store, how many times a difference
        // pair has occurred previously (key = diff1 *
        // diff2)
        HashMap<String, Integer> map = new HashMap<>();
        map.put("0*0", 1);

        //  zc (Count of zeroes), oc(Count of 1s)
        //  and tc(count of twos)
        //  In starting all counts are zero
        int zc = 0, oc = 0, tc = 0;

        int ans = 0;

        //  looping into string
        for (int i = 0; i < str.length(); i++) {

            // Increasing the count of current character
            if (str.charAt(i) == '0')
                zc++;
            else if (str.charAt(i) == '1')
                oc++;
            else
                tc++;

            // making key of differences (z[i] - o[i],
            // z[i] - t[i])
            String key = (zc - oc) + "*" + (zc - tc);

            // Count of previous occurrences of above pair
            // indicates that the subarrays forming from
            // every previous occurrence to this occurrence
            // is a subarray with equal number of 0's, 1's
            // and 2's
            ans += map.getOrDefault(key, 0);

            map.put(key, map.getOrDefault(key, 0) + 1);
        }

        // increasing the count of current difference
        // pair by 1
        return ans;
    }

    // Driver's Code
    public static void main(String[] args)
    {
        String str = "0102010";

          // Function call
        System.out.println(getSubstringWithEqual012(str));
    }
}

Python3

# Python3 program to find substring with equal
# number of 0's, 1's and 2's

# Method to count number of substring which
# has equal 0, 1 and 2
def getSubstringWithEqual012(string):
    N = len(string)

    # map to store, how many times a difference
    # pair has occurred previously
    mp = dict()
    mp[(0, 0)] = 1

    # zc (Count of zeroes), oc(Count of 1s)
    # and tc(count of twos)
    # In starting all counts are zero
    zc, oc, tc = 0, 0, 0

    # looping into string
    res = 0 # Initialize result
    for i in range(N):

        # increasing the count of current character
        if string[i] == '0':
            zc += 1
        elif string[i] == '1':
            oc += 1
        else:
            tc += 1 # Assuming that string doesn't contain
                    # other characters

        # making pair of differences (z[i] - o[i],
        # z[i] - t[i])
        tmp = (zc - oc, zc - tc)

        # Count of previous occurrences of above pair
        # indicates that the subarrays forming from
        # every previous occurrence to this occurrence
        # is a subarray with equal number of 0's, 1's
        # and 2's
        if tmp not in mp:
            res += 0
        else:
            res += mp[tmp]

        # increasing the count of current difference
        # pair by 1
        if tmp in mp:
            mp[tmp] += 1
        else:
            mp[tmp] = 1

    return res

# Driver's Code
if __name__ == "__main__":
    string = "0102010"
    print(getSubstringWithEqual012(string))

# This code is contributed by
# sanjeev2552

Output

2

Time Complexity: O(N * log N)
Auxiliary Space: O(N)

This article is contributed by Utkarsh Trivedi. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.