[0018 fourSum]add C version

add C version for 0018: fourSum problem

Signed-off-by: liao junwu <andyliaowu5@163.com>

problems/0018.四数之和.md

@@ -151,6 +151,96 @@ if (nums[k] + nums[i] > target && nums[i] >= 0) {
 
 ## 其他语言版本
 
+### C:
+
+```C
+/* qsort */
+static int cmp(const void* arg1, const void* arg2) {
+    int a = *(int *)arg1;
+    int b = *(int *)arg2;
+    return (a > b);
+}
+
+int** fourSum(int* nums, int numsSize, int target, int* returnSize, int** returnColumnSizes) {
+
+    /* 对nums数组进行排序 */
+    qsort(nums, numsSize, sizeof(int), cmp);
+
+    int **res = (int **)malloc(sizeof(int *) * 40000);
+    int index = 0;
+
+    /* k */
+    for (int k = 0; k < numsSize - 3; k++) { /* 第一级 */
+
+        /* k剪枝 */
+        if ((nums[k] > target) && (nums[k] >= 0)) {
+            break;
+        }
+        /* k去重 */
+        if ((k > 0) && (nums[k] == nums[k - 1])) {
+            continue;
+        }
+
+        /* i */
+        for (int i = k + 1; i < numsSize - 2; i++) { /* 第二级 */
+
+            /* i剪枝 */
+            if ((nums[k] + nums[i] > target) && (nums[i] >= 0)) {
+                break;
+            }
+            /* i去重 */
+            if ((i > (k + 1)) && (nums[i] == nums[i - 1])) {
+                continue;
+            }
+
+            /* left and right */
+            int left = i + 1;
+            int right = numsSize - 1;
+
+            while (left < right) {
+
+                /* 防止大数溢出 */
+                long long val = (long long)nums[k] + nums[i] + nums[left] + nums[right];
+                if (val > target) {
+                    right--;
+                } else if (val < target) {
+                    left++;
+                } else {
+                    int *res_tmp = (int *)malloc(sizeof(int) * 4);
+                    res_tmp[0] = nums[k];
+                    res_tmp[1] = nums[i];
+                    res_tmp[2] = nums[left];
+                    res_tmp[3] = nums[right];
+                    res[index++] = res_tmp;
+                    
+                    /* right去重 */
+                    while ((right > left) && (nums[right] == nums[right - 1])) {
+                        right--;
+                    }
+                    /* left去重 */
+                    while ((left < right) && (nums[left] == nums[left + 1])) {
+                        left++;
+                    }
+
+                    /* 更新right与left */
+                    left++, right--;
+                }
+            }
+        }
+    }
+
+    /* 返回值处理 */
+    *returnSize = index;
+
+    int *column = (int *)malloc(sizeof(int) * index);
+    for (int i = 0; i < index; i++) {
+        column[i] = 4;
+    }
+    *returnColumnSizes = column;
+    return res;
+}
+```
+
 ### Java：
 
 ```Java