Separate Black and White Balls

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. 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_.   **Example 1:** **Input:** s = "101" **Output:** 1 **Explanation:** We can group all the black balls to the right in the following way: - Swap s[0] and s[1], s = "011". Initially, 1s are not grouped together, requiring at least 1 step to group them to the right. ``` **Example 2:** **Input:** s = "100" **Output:** 2 **Explanation:** We can group all the black balls to the right in the following way: - Swap s[0] and s[1], s = "010". - Swap s[1] and s[2], s = "001". It can be proven that the minimum number of steps needed is 2. ``` **Example 3:** **Input:** s = "0111" **Output:** 0 **Explanation:** All the black balls are already grouped to the right. ```   **Constraints:** - `1 <= n == s.length <= 105` - `s[i]` is either `'0'` or `'1'`.