链接: 三步问题

编写代码

class Solution {
public:
    int waysToStep(int n) {
        if(n == 1 || n == 2) return n;
        vector<int> dp(n+1);
        const int MOD = 1e9 + 7;
        dp[0] = dp[1] = 1;
        dp[2] = 2;
        for(int i = 3;i<=n;++i)
        {
            dp[i] = ((dp[i - 1] + dp[i - 2]) % MOD + dp[i - 3]) % MOD;
        }
        return dp[n];
    }
};

代码优化

class Solution {
public:
    int waysToStep(int n) {
        if(n == 1 || n == 2) return n;
        int a = 1,b = 2,c = 4,d = 0;
        const int MOD = 1e9 + 7;
        for(int i = 4;i<=n;++i)
        {
            d = ((b + c) % MOD + a) % MOD;
            a = b;
            b = c;
            c = d;
        }
        return c;
    }
};