LeetCode 1864: Minimum Number of Swaps to Make the Binary String Alternating Solution

Master LeetCode problem 1864 (Minimum Number of Swaps to Make the Binary String Alternating), a medium challenge, with our optimized solutions in Java, C++, and Python. Explore detailed explanations, test your code in our interactive editor, and prepare for coding interviews.

1864. Minimum Number of Swaps to Make the Binary String Alternating

Medium

String Greedy

Problem Explanation

Explanation

To solve this problem, we need to check if it is possible to make the binary string alternating by swapping characters. We can do this by counting the number of zeros and ones in the string and then analyzing the possible cases. If the absolute difference between the counts of zeros and ones is at most 1, we can determine the minimum number of swaps required.

If the string length is odd, we have two cases:
- Case 1: The string starts with '0'. In this case, we need to count how many swaps are needed to make the string "0101...". This can be done by comparing the count of '0's at even indices and '1's at odd indices with the actual count.
- Case 2: The string starts with '1'. In this case, we need to count how many swaps are needed to make the string "1010...".
If the string length is even, we have only one case:
- We need to find the minimum swaps required to make the string "0101..." or "1010...".

If the minimum swaps required exceed half of the string length, it means it is impossible to make the string alternating.

Time Complexity: O(n), where n is the length of the input string. Space Complexity: O(1)

Solution Code

class Solution {
    public int minSwaps(String s) {
        int n = s.length();
        int countZero = 0, countOne = 0;
        
        for(char c : s.toCharArray()) {
            if(c == '0') countZero++;
            else countOne++;
        }
        
        if(Math.abs(countZero - countOne) > 1) return -1;
        
        int minSwaps = 0;
        if(n % 2 == 0) {
            minSwaps = Math.min(countSwaps(s, '0'), countSwaps(s, '1'));
        } else {
            if(countZero > countOne) minSwaps = countSwaps(s, '0');
            else minSwaps = countSwaps(s, '1');
        }
        
        return minSwaps / 2;
    }
    
    private int countSwaps(String s, char startChar) {
        int swaps = 0;
        for(int i = 0; i < s.length(); i++) {
            if(s.charAt(i) != startChar) swaps++;
            startChar = startChar == '0' ? '1' : '0';
        }
        return swaps;
    }
}

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