# Find minimum shift for longest common prefix in C++

C++Server Side ProgrammingProgramming

Consider we have two strings A and B. The length of A and B are same. In a single shift we can rotate the string B one element. We have to find minimum required shift to get common prefix of maximum length from A and B. So if A = “computerprogramming”, and B = “programminglanguage” So the minimum shift is 8, and prefix is “programming”.

Suppose we add the string B at the end of B, so B = B + B, then there is no-need to find prefix of each shift separately. So we have to find the longest prefix of A present in B, and the starting position of the prefix in B, will give the actual number of required shifts.

## Example

Live Demo

#include<iostream>
using namespace std;
void KhuthMorrisPatt(int m, int n, string B, string A) {
int pos = 0, len = 0;
int p[m + 1];
int k = 0;
p = 0;
for (int i = 2; i <= n; i++) {
while (k > 0 && A[k] != A[i - 1])
k = p[k];
if (A[k] == A[i - 1])
++k;
p[i] = k;
}
for (int j = 0, i = 0; i < m; i++) {
while (j > 0 && A[j] != B[i])
j = p[j];
if (A[j] == B[i])
j++;
if (j > len) {
len = j;
pos = i - j + 1;
}
}
cout << "Shift = " << pos << endl;
cout << "Prefix = " << A.substr(0, len);
}
int main() {
string A = "programminglanguage";
string B = "computerprogramming";
int n = A.size();
B = B + B;
KhuthMorrisPatt(2 * n, n, B, A);
}

## Output

Shift = 8
Prefix = programming