Merge pull request codemistic#466 from Aman5989/patch-14

create find-substring-with-given-hash-value.cpp

Author: gantavyamalviya

Hashtable/Find-Substring-With-Given-Hash-Value.cpp

@@ -0,0 +1,21 @@
+class Solution {
+public:
+ string subStrHash(string s, int p, int mod, int k, int target) {
+ long h = 0, N = s.size(), pp = 1; // `pp` = p^k
+ vector<long> hash(N);
+ string r(rbegin(s), rend(s));
+ for (int i = 0; i < N; ++i) {
+ if (i < k) pp = pp * p % mod;
+ h = (h * p + (r[i] - 'a' + 1)) % mod; // push r[i] into the window
+ if (i - k >= 0) { // pop r[i-k] out of the window
+ h = (h - (r[i - k] - 'a' + 1) * pp % mod + mod) % mod;
+ }
+ if (i >= k - 1) hash[i] = h;
+ }
+ reverse(begin(hash), end(hash));
+ for (int i = 0; i < N; ++i) {
+ if (hash[i] == target) return s.substr(i, k); // hash[i] is the hash of `s[i .. (i+k-1)]`
+ }
+ return "";
+ }
+};