KMP algorithm implementation for string search

leetcode/README.md

@@ -13,6 +13,7 @@ LeetCode
 |24|[Swap Nodes in Pairs](https://leetcode.com/problems/swap-nodes-in-pairs/) | [C](./src/24.c)|Medium|
 |26|[Remove Duplicates from Sorted Array](https://leetcode.com/problems/remove-duplicates-from-sorted-array/) | [C](./src/26.c)|Easy|
 |27|[Remove Element](https://leetcode.com/problems/remove-element/) | [C](./src/27.c)|Easy|
+|28|[Implement strStr()](https://leetcode.com/problems/implement-strstr/) | [C](./src/28.c)|Easy|
 |35|[Search Insert Position](https://leetcode.com/problems/search-insert-position/) | [C](./src/35.c)|Easy|
 |53|[Maximum Subarray](https://leetcode.com/problems/maximum-subarray/) | [C](./src/53.c)|Easy|
 |82|[Remove Duplicates from Sorted List II](https://leetcode.com/problems/remove-duplicates-from-sorted-list-ii/) | [C](./src/82.c)|Medium|
@@ -73,4 +74,4 @@ LeetCode
 |1089|[Duplicate Zeros](https://leetcode.com/problems/duplicate-zeros/) | [C](./src/1089.c)|Easy|
 |1184|[Distance Between Bus Stops](https://leetcode.com/problems/distance-between-bus-stops/) | [C](./src/1184.c)|Easy|
 |1189|[Maximum Number of Balloons](https://leetcode.com/problems/maximum-number-of-balloons/) | [C](./src/1189.c)|Easy|
-|1207|[Unique Number of Occurrences](https://leetcode.com/problems/unique-number-of-occurrences/) | [C](./src/1207.c)|Easy|
+|1207|[Unique Number of Occurrences](https://leetcode.com/problems/unique-number-of-occurrences/) | [C](./src/1207.c)|Easy|

leetcode/src/28.c

@@ -0,0 +1,104 @@
+/* 
+ * brute force approach
+ * time complexity: O(mn)
+ */
+int strStr(char* haystack, char* needle) {
+    int i = 0;
+    int j = 0;
+    int k = 0;
+    int hlen = 0;
+    int nlen = 0;
+
+    if(needle == NULL || *needle == 0)
+        return 0;
+
+    if(haystack == NULL || *haystack == 0)
+        return -1;
+
+    hlen = strlen(haystack);
+    nlen = strlen(needle);
+
+    if(hlen < nlen)
+        return -1;
+
+    for(i = 0; i <= hlen - nlen; i++) {  
+        j = 0;
+        if(haystack[i] != needle[j++])
+            continue;
+
+        k = i + 1;
+        for(; j < nlen; j++) {
+            if(haystack[k] != needle[j]) {
+                break;
+            } else
+                k++;
+        }
+        if(j == nlen)
+            return i;
+    }
+    return -1;
+}
+
+/* ---------------------------------------------------------------------------------------- */
+
+/*
+ * KMP algorithm
+ * time complexity: O(m + n)
+ */
+
+/* fills overlap with longest proper prefix which is also suffix for each index in needle */
+void fill_overlap(char *needle, int len_needle, int *overlap)
+{
+    int len = 0;
+    int i = 0;
+
+    overlap[0] = 0;
+
+    for (i = 1; i < len_needle;) {
+        if (needle[i] == needle[len]) {
+            len++;
+            overlap[i++] = len;
+        } else {
+            if (len)
+                len = overlap[len - 1];
+            else
+                overlap[i++] = 0;
+        }
+    }
+}
+
+int strStr(char *haystack, char *needle)
+{
+    int i = 0; /* index for haystack */
+    int j = 0; /* index for needle */
+
+    int len_needle = strlen(needle);
+    int len_haystack = strlen(haystack);
+
+    if (!len_needle)
+        return 0;
+
+    int overlap[len_needle];
+
+    fill_overlap(needle, len_needle, overlap);
+
+    while (i < len_haystack) {
+        if (needle[j] == haystack[i]) {
+            i++;
+            j++;
+        }
+
+        if (j == len_needle) {
+            return (i - j);
+        } else if (i < len_haystack && needle[j] != haystack[i]) {
+            if (j != 0)
+                j = overlap[j - 1];
+            else
+                i = i + 1;
+        }
+    }
+    return -1;
+}
+
+/* ---------------------------------------------------------------------------------------- */
+