add kmp.cpp

C++/Algorithms/StingMatching/kmp.cpp

@@ -0,0 +1,60 @@
+/* This program prints the number of times given pattern occurs in a string in O(n) using KMP */
+#include<bits/stdc++.h>
+using namespace std;
+typedef long long ll;
+typedef unsigned long long ull;
+#define mod 1000000007
+#define FOR(i,x) for(int i = 0; i < x; i++)
+#define For(x,y) for(int i = x; i <= y; i++)
+#define vint(x) vector<int>x
+#define vl(x) vector<long>x
+#define vll(x) vector<long long>x
+#define pb push_back
+#define oset tree <int, null_type , less , rb_tree_tag, tree_order_statistics_node_update>
+const int mx = 1e6;
+int main()
+{
+//	freopen("input.txt", "r", stdin);
+//	freopen("output.txt", "w", stdout);
+	ios_base::sync_with_stdio(0);
+	cin.tie(0);
+	cout.tie(0);
+    string p,s;
+    cin>>p>>s;
+    int n = s.length(), m = p.length(), ans = 0;
+    int lps[m] = {0};   // An array of longest prefix count for each index
+    for(int i = 1; i < m; i++)   // Calculating longest proper prefix which is also a suffix
+    {
+        int j = lps[i-1];
+        while(j > 0 && p[i] != p[j])
+        j = lps[j-1];
+        if(p[i] == p[j])
+        ++j;
+        lps[i] = j;
+    }
+    int j = 0;
+    for(int i = 0; i <= (n-m); i++)
+    {
+        int q = i;
+        for(; j < m; j++)
+        {
+            if(p[j] == s[i])
+            ++i;
+            else
+            break;
+        }
+        if(j == m) // Pattern Matches
+        {
+            ++ans;
+            i = q;
+        }
+        else
+        {
+            if(j == 0) // If first character itself don't match
+            continue;
+            --i;      // Start matching again at mismatch character
+        }
+        j = lps[j-1];
+    }
+    cout<<ans<<endl;
+}