Update README.md

Author: yanglbme

solution/0000-0099/0001.Two Sum/README.md

@@ -353,6 +353,55 @@ class Solution {
 }
 ```
 
+#### C
+
+```c
+#include <stdlib.h>
+
+int* twoSum(int* nums, int numsSize, int target, int* returnSize) {
+    int capacity = 1;
+    while (capacity < numsSize * 2) capacity <<= 1;
+    int* keys = malloc(capacity * sizeof(int));
+    int* vals = malloc(capacity * sizeof(int));
+    char* used = calloc(capacity, sizeof(char));
+    if (!keys || !vals || !used) {
+        free(keys);
+        free(vals);
+        free(used);
+        *returnSize = 0;
+        return NULL;
+    }
+    for (int i = 0; i < numsSize; ++i) {
+        int x = nums[i];
+        int y = target - x;
+        unsigned int h = (unsigned int) y & (capacity - 1);
+        while (used[h]) {
+            if (keys[h] == y) {
+                int* res = malloc(2 * sizeof(int));
+                res[0] = vals[h];
+                res[1] = i;
+                *returnSize = 2;
+                free(keys);
+                free(vals);
+                free(used);
+                return res;
+            }
+            h = (h + 1) & (capacity - 1);
+        }
+        unsigned int h2 = (unsigned int) x & (capacity - 1);
+        while (used[h2]) h2 = (h2 + 1) & (capacity - 1);
+        used[h2] = 1;
+        keys[h2] = x;
+        vals[h2] = i;
+    }
+    *returnSize = 0;
+    free(keys);
+    free(vals);
+    free(used);
+    return NULL;
+}
+```
+
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