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Author: MishkatIT

0028-find-the-index-of-the-first-occurrence-in-a-string/0028-find-the-index-of-the-first-occurrence-in-a-string.cpp

@@ -1,11 +1,47 @@
+
+void computeLPS(int m, string& ptr, vector<int>& lps) {
+    int len = 0, i = 1;
+    while (i < m) {
+        if (ptr[i] == ptr[len]) {
+            len++;
+            lps[i] = len;
+            i++;
+        } else {
+            if (len != 0) {
+                len = lps[len - 1];
+            } else {
+                i++;
+            }
+        }
+    }
+}
+
+
+int KMP(string& str, string& ptr) {
+    int n = str.length(), m = ptr.length();
+    vector<int> lps(m);
+    computeLPS(m, ptr, lps);
+    int i = 0, j = 0;
+    while (i < n) {
+        if (str[i] == ptr[j]) {
+            i++, j++;
+        } else {
+            if (j != 0) {
+                j = lps[j - 1];
+            } else {
+                i++;
+            }
+        }
+        if (j == m) {
+            return (i - j);
+        }
+    }
+    return -1;
+}
+
 class Solution {
 public:
     int strStr(string haystack, string needle) {
-        auto pos = haystack.find(needle);
-        if(pos != string::npos) {
-            return pos;
-        } else {
-            return -1;
-        }
+        return KMP(haystack, needle);
     }
 };