排列组合

排列实现

public int arrange(int n, int r) {
    if (n < r || n < 0 || r < 0) {
        return -1;
    }
    if (n == r) {
        return 1;
    }
    //从n个不同元素中取r个按序排列
    long molecular = 1, denominator = n - r;
    int count = n;
    while (count > 0) {
        molecular *= (count--);
        while (denominator > 0 && molecular % denominator == 0) {
            molecular /= denominator;
            denominator--;
        }
    }
    return (int) molecular;
}

组合实现

public static int combine(int n, int r) {
        if (n < r || n < 0 || r < 0) {
            return -1;
        }
        if (n == r) {
            return 1;
        }
        r = Math.min(n - r, r);
        //分子
        long molecular = 1;
        //分母  消去部分分母的乘数个数count 从t开始向下count个
        int denominator = r, count = r, t = n;
        while (count-- > 0) {
            molecular *= (t--);
            //计算的时候就进行除法，防止溢出
            while (denominator > 0 && molecular % denominator == 0) {
                molecular /= denominator;
                denominator--;
            }
        }
        return (int) molecular;
}

注意事项

• 处理溢出，别越界！！！！！！！！