力扣2156.查找给定哈希值的子串

• rolling hash：求带权的值 左边是高位 右边是低位

  • 本题要求左边低位 只要反向求即可

•   class Solution {public:string subStrHash(string s, int power, int modulo, int k, int hashValue) {int n = s.size();long long M = modulo,pk=1,hash = 0;//预处理k个 并求出p^k-1for(int i=n-1;i>=n-k;i--){hash = ((long long)hash*power + (s[i] - 'a' + 1)) % M;if(i != n-k) pk = (long long)pk*power%M;}int pos = -1;if(hash == hashValue) pos = n-k;for(int i=n-k-1;i>=0;i--){//把右边高位减掉hash = hash - (s[i+k] - 'a' + 1) * pk % M;//防止hash减完成负数hash = (hash + M) %M;//整体左移(升高一位)hash = hash * power % M;//加上左边低位hash = (hash + s[i]-'a'+1) % M;if(hash == hashValue){pos = i;}}return s.substr(pos,k);}};