Algorithm

Leetcode 1830. Minimum Number of Operations to Make String Sorted

挺神秘的一个题，题意自己看吧 https://leetcode.com/problems/minimum-number-of-operations-to-make-string-sorted/

先写一个暴力做法，然后找规律。需要用到快速幂。太麻烦了，懒得讲了。

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class Solution {
    int ksm(int a, int b, int c) {
        int res = 1;
        while (b > 0) {
            if (b & 1) res = (long long)res * a % c;
            a = (long long) a * a % c;
            b >>= 1;
        }
        return res;
    }

    int get_ans(string s) {
        int n = s.length();
        int mo = 1000000007;
        vector<int> f(n + 1);
        vector<int> ni(n + 1);
        f[0] = 1;
        f[1] = 1;
        ni[0] = 1;
        ni[1] = 1;
        for (int i = 2; i <= n; i++) {
            f[i] = (long long) f[i - 1] * i % mo;
            ni[i] = ksm(f[i], mo - 2, mo);
        }
        int ans = 0;
        vector<int> num(26);
        num[s[s.length() - 1] - 'a']++;
        for (int now = s.length() - 2; now >= 0; now--) {
            num[s[now] - 'a']++;
            int tmp = f[s.length() - now - 1];
            for (int j = 0; j < 26; j++) {
                tmp = (long long) tmp * ni[num[j]] % mo;
            }
            for (int i = s[now] - 'a' - 1; i >= 0; i--) {
                if (num[i] == 0) continue;
                int qq = (long long) tmp * num[i] % mo;
                ans = (ans + qq) % mo;
            }
        }
        return ans % mo;
    }

public:
    int makeStringSorted(string s) {
        return get_ans(s);
    }
};