Separate Black and White Balls — LeetCode 2938 Python Solution

The problem

There are n balls on a table, each ball has a color black or white. You are given a 0-indexed binary string s of length n, where 1 and 0 represent black and white balls, respectively.

Example

Input

s = "101"

Output

1

Explanation

We can group all the black balls to the right in the following way:

Python solution

class Solution:
    def minimumSteps(self, s: str) -> int:
        n = len(s)
        ans = cnt = 0
        for i in range(n - 1, -1, -1):
            if s[i] == '1':
                cnt += 1
                ans += n - i - cnt
        return ans

Complexity

Pattern: Two Pointers

Use the order already in the input to discard half the search space at every step. LeetCode 2938. Separate Black and White Balls is filed here because LeetCode tags it Two Pointers, which is the vocabulary this hub collects.

The two pointers guide has the Python template for the pattern and the 201 LeetCode problems that use it.

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