差分

https://leetcode.cn/problems/shifting-letters-ii/

class Solution {
public:
    string shiftingLetters(string s, vector<vector<int>>& shifts) {
        int n = s.size();
        vector<int> f(n + 1);
        /* 差分数组 */
        for (auto &shift : shifts) {
            int x = shift[2] * 2 - 1;
            f[shift[0]]     += x;
            f[shift[1] + 1] -= x;
        }

        int sum = 0;
        string ans;
        for (int i = 0; i < n; i++) { /* 前缀和求出最终的字符 */
            sum += f[i];
            int d = ((s[i] - 'a' + sum) % 26 + 26) % 26;
            ans.push_back(d + 'a');
        }
        return ans;
    }
};