Separate black and white balls

Two Pointers String Greedy

64.0% acceptance

Feb 25, 2026

878

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. In each step, you can choose two adjacent balls and swap them. Return the minimum number of steps to group all the black balls to the right and all the white balls to the left.

Solution

Rust

Time O(n)

Space O(1)

LeetCode

solution.rs

impl Solution {
  pub fn minimum_steps(s: String) -> i64 {
    let mut ones_before = 0i64;
    let mut ans = 0i64;
    for ch in s.bytes() {
      if ch == b'1' {
        ones_before += 1;
      } else {
        ans += ones_before;
      }
    }
    ans
  }
}