题目描述

题解思路

题目可以转换成爬楼梯，即从第零层开始，每次能够向上爬zero层或者向上爬one层，最终能够爬到[low, high]层的种数总和
第n层是从第n - zero层或者第n - one层爬上来的，所以我们从最底层开始向上爬，记录过程中爬到每一层的方案数
当爬到[low, high]层内的时候进行结果计数

题解代码

impl Solution {
    pub fn count_good_strings(low: i32, high: i32, zero: i32, one: i32) -> i32 {
        let m = 1_000_000_007;
        let mut f = vec![0; high as usize + 1];
        f[0] = 1;
        let mut ans = 0;
        for i in 1..=high {
            if i >= zero {
                f[i as usize] = f[(i - zero) as usize];
            }
            if i >= one {
                f[i as usize] = (f[i as usize] + f[(i - one) as usize]) % m;
            }
            if i >= low {
                ans = (ans + f[i as usize]) % m;
            }
        }
        ans
    }
}

题目链接

https://leetcode.cn/problems/count-ways-to-build-good-strings/