[pull] master from youngyangyang04:master

problems/0454.四数相加II.md

@@ -412,6 +412,75 @@ public int FourSumCount(int[] nums1, int[] nums2, int[] nums3, int[] nums4) {
         return res;
     }
 ```
+### C：
+
+```c
+// 哈希表大小
+const int HASH_SIZE = 101;
+
+typedef struct node {
+    int val;
+    int count;
+    struct node *next;
+} node, *HashMap;
+
+// 哈希表插入
+void hash_insert(HashMap hashmap[], int val) {
+    int idx = val < 0 ? (-val) % HASH_SIZE : val % HASH_SIZE, count = 0;
+    node *p = hashmap[idx];
+    while (p->next != NULL) {
+        p = p->next;
+        if (p->val == val) {
+            (p->count)++;
+            return;
+        }
+    }
+    node *new = malloc(sizeof(node));
+    new->val = val;
+    new->count = 1;
+    new->next = NULL;
+    p->next = new;
+    return;
+}
+
+// 哈希表查找
+int hash_search(HashMap hashmap[], int val) {
+    int idx = val < 0 ? (-val) % HASH_SIZE : val % HASH_SIZE;
+    node *p = hashmap[idx];
+    while (p->next != NULL) {
+        p = p->next;
+        if (p->val == val) return p->count;
+    }
+    return 0;
+}
+
+int fourSumCount(int* nums1, int nums1Size, int* nums2, int nums2Size, int* nums3, int nums3Size, int* nums4, int nums4Size){
+    // 初始化哈希表
+    HashMap hashmap[HASH_SIZE];
+    for (int i = 0; i < HASH_SIZE; i++) {
+        hashmap[i] = malloc(sizeof(node));
+        hashmap[i]->next = NULL;
+    }
+
+    // 统计两个数组元素之和的负值和出现的次数，放到哈希表中
+    int count = 0, num;
+    for (int i = 0; i < nums1Size; i++) {
+        for(int j = 0; j < nums2Size; j++) {
+            num = - nums1[i] - nums2[j];
+            hash_insert(hashmap, num);
+        }
+    }
+
+    // 统计另外两个数组元素之和，查找哈希表中对应元素的出现次数，加入总次数
+    for (int i = 0; i < nums3Size; i++) {
+        for(int j = 0; j < nums4Size; j++) {
+            num = nums3[i] + nums4[j];
+            count += hash_search(hashmap, num);
+        }
+    }
+    return count;
+}
+```
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