diff --git a/leetcode/src/30.c b/leetcode/src/30.c
new file mode 100644
index 0000000000..bb83be0bab
--- /dev/null
+++ b/leetcode/src/30.c
@@ -0,0 +1,70 @@
+
+void cm(void* ptr, size_t size) {
+    unsigned char* p = (unsigned char*)ptr;
+    while (size--) {
+        *p++ = 0;
+    }
+}
+
+/**
+ * Return an array of starting indices of substring(s) in `s` that are a concatenation 
+ * of each word in `words` exactly once, in any order. 
+ * The sizes of the arrays are returned as *returnSize array. 
+ * Note: The returned array must be malloced, assume caller calls free().
+ */
+int* findSubstring(char* s, char** w, int ws, int* rs) {
+    int nw = 0, tl = 1, wl = strlen(w[0]), sl = strlen(s);
+    int count[ws + 1], map[wl * ws + 1], t[wl * ws][26];
+    
+    cm(count, sizeof(count));
+    cm(map, sizeof(map));
+    cm(t, sizeof(t));
+
+    for (int i = 0; i < ws; i++) {
+        int n = 0;
+        for (int j = 0; j < wl; j++) {
+            char nIdx = w[i][j] - 'a';
+            if (t[n][nIdx] == 0) t[n][nIdx] = tl++;
+            n = t[n][nIdx];
+        }
+        if (map[n] == 0) map[n] = ++nw;
+        count[map[n]]++;
+    }
+
+    int* result = (int*)malloc(sl * sizeof(int));
+    int resLen = 0;
+    int m[sl / wl + 1], wCount[nw + 1];
+
+    for (int i = 0; i < wl; i++) {
+        if (i + wl * ws > sl) continue;
+        cm(wCount, sizeof(wCount));
+        int h = 0, mCount = 0;
+
+        for (int j = i, k = 0, l = 0; j + wl <= sl; j += wl, k++) {
+            int n = 0;
+            for (int mIdx = j; mIdx < j + wl; mIdx++) {
+                n = t[n][s[mIdx] - 'a'];
+                if (n == 0) { mCount++; m[k] = -1; break; }
+            }
+            if (n != 0) {
+                int cIdx = map[n];
+                m[k] = cIdx;
+                wCount[cIdx]++;
+                if (wCount[cIdx] == count[cIdx]) h++;
+                else while (wCount[cIdx] > count[cIdx]) {
+                    int leftIdx = m[l++];
+                    if (leftIdx == -1) mCount--;
+                    else {
+                        if (wCount[leftIdx] == count[leftIdx]) h--;
+                        wCount[leftIdx]--;
+                    }
+                }
+            }
+            while (l <= k && m[l] == -1) { l++; mCount--; }
+            if (h == nw && mCount == 0) result[resLen++] = i + l * wl;
+        }
+    }
+
+    *rs = resLen;
+    return result;
+}