分割字符串的方案数

/*
 * 使用 Java Stream 的解法：
 * 1. 计算总的 '1' 的数量。
 * 2. 如果不是 3 的倍数，返回 0。
 * 3. 通过流操作找到分割点并计算组合数。
 */

import java.util.stream.IntStream;

public class Solution {
    public int numWays(String s) {
        int MOD = 1000000007;
        int totalOnes = (int)s.chars().filter(c -> c == '1').count();
        if (totalOnes % 3 != 0) return 0;
        if (totalOnes == 0) return (int)(((long)(s.length() - 1) * (s.length() - 2) / 2) % MOD);

        int onesPerPart = totalOnes / 3;
        int[] counts = {0, 0, 0}; // {count, first, second}
        int onesSoFar = 0;

        for (char c : s.toCharArray()) {
            if (c == '1') onesSoFar++;
            if (onesSoFar == onesPerPart) counts[0]++;
            if (onesSoFar == onesPerPart + 1) counts[1]++;
            if (onesSoFar == 2 * onesPerPart) counts[2]++;
        }

        return (int)(((long)counts[1] * (long)counts[2]) % MOD);
    }
}