added code of kmp algorithm in index.md (#22293)

* added code of kmp algorithm in index.md

* format codeblock to c++?

I think this is c++, but I'm not positive.

guide/english/algorithms/string-matching-algorithms/kmp-algorithm/index.md

@@ -53,6 +53,83 @@ How to use lps[] to decide next positions (or to know a number of characters to
     -   From above two points, we can conclude that we do not need to match these lps[j-1] characters with txt[i-j…i-1] because we know that these characters will anyway match. Let us consider above example to understand this.
 <br>
 
+### Code Of KMP Algorithm in C++
+```cpp
+void computeLPSArray(char* pat, int M, int* lps) 
+{ 
+    // length of the previous longest prefix suffix 
+    int len = 0; 
+  
+    lps[0] = 0; // lps[0] is always 0 
+  
+    // the loop calculates lps[i] for i = 1 to M-1 
+    int i = 1; 
+    while (i < M) { 
+        if (pat[i] == pat[len]) { 
+            len++; 
+            lps[i] = len; 
+            i++; 
+        } 
+        else // (pat[i] != pat[len]) 
+        { 
+            // This is tricky. Consider the example. 
+            // AAACAAAA and i = 7. The idea is similar 
+            // to search step. 
+            if (len != 0) { 
+                len = lps[len - 1]; 
+  
+                // Also, note that we do not increment 
+                // i here 
+            } 
+            else // if (len == 0) 
+            { 
+                lps[i] = 0; 
+                i++; 
+            } 
+        } 
+    } 
+} 
+```
+
+```cpp
+void KMPSearch(char* pat, char* txt) 
+{ 
+    int M = strlen(pat); 
+    int N = strlen(txt); 
+  
+    // create lps[] that will hold the longest prefix suffix 
+    // values for pattern 
+    int lps[M]; 
+  
+    // Preprocess the pattern (calculate lps[] array) 
+    computeLPSArray(pat, M, lps); 
+  
+    int i = 0; // index for txt[] 
+    int j = 0; // index for pat[] 
+    while (i < N) { 
+        if (pat[j] == txt[i]) { 
+            j++; 
+            i++; 
+        } 
+  
+        if (j == M) { 
+            printf("Found pattern at index %d ", i - j); 
+            j = lps[j - 1]; 
+        } 
+  
+        // mismatch after j matches 
+        else if (i < N && pat[j] != txt[i]) { 
+            // Do not match lps[0..lps[j-1]] characters, 
+            // they will match anyway 
+            if (j != 0) 
+                j = lps[j - 1]; 
+            else
+                i = i + 1; 
+        } 
+    } 
+} 
+```
+
 **More Infromation :**
 - [kmp algorithm for pattern searching](https://www.geeksforgeeks.org/kmp-algorithm-for-pattern-searching/)
 - [Knuth–Morris–Pratt algorithm](https://en.wikipedia.org/wiki/Knuth%E2%80%93Morris%E2%80%93Pratt_algorithm)