优化0332重新安排行程 C 版本

遍历所有ticket查找机场为O(n), 修改为使用hash表查找.

Author: slaier

problems/0332.重新安排行程.md

@@ -652,62 +652,100 @@ function findItinerary(tickets: string[][]): string[] {
 ### C
 
 ```C
-char **result;
-bool *used;
-int g_found;
-
-int cmp(const void *str1, const void *str2)
-{
-    const char **tmp1 = *(char**)str1;
-    const char **tmp2 = *(char**)str2;
-    int ret = strcmp(tmp1[0], tmp2[0]);
-    if (ret == 0) {
-        return strcmp(tmp1[1], tmp2[1]);
+typedef struct {
+    char *name;        /* key */
+    int cnt;           /* 记录到达机场是否飞过了 */
+    UT_hash_handle hh; /* makes this structure hashable */
+} to_airport_t;
+
+typedef struct {
+    char *name; /* key */
+    to_airport_t *to_airports;
+    UT_hash_handle hh; /* makes this structure hashable */
+} from_airport_t;
+
+void to_airport_destroy(to_airport_t *airports) {
+    to_airport_t *airport, *tmp;
+    HASH_ITER(hh, airports, airport, tmp) {
+        HASH_DEL(airports, airport);
+        free(airport);
     }
-    return ret;
 }
 
-void backtracting(char *** tickets, int ticketsSize, int* returnSize, char *start, char **result, bool *used)
-{
-    if (*returnSize == ticketsSize + 1) {
-        g_found = 1;
-        return;
+void from_airport_destroy(from_airport_t *airports) {
+    from_airport_t *airport, *tmp;
+    HASH_ITER(hh, airports, airport, tmp) {
+        to_airport_destroy(airport->to_airports);
+        HASH_DEL(airports, airport);
+        free(airport);
+    }
+}
+
+int name_sort(to_airport_t *a, to_airport_t *b) {
+    return strcmp(a->name, b->name);
+}
+
+bool backtracking(from_airport_t *airports, int target_path_len, char **path,
+                  int path_len) {
+    if (path_len == target_path_len) return true;
+
+    from_airport_t *from_airport = NULL;
+    HASH_FIND_STR(airports, path[path_len - 1], from_airport);
+    if (!from_airport) return false;
+
+    for (to_airport_t *to_airport = from_airport->to_airports;
+         to_airport != NULL; to_airport = to_airport->hh.next) {
+        if (to_airport->cnt == 0) continue;
+        to_airport->cnt--;
+        path[path_len] = to_airport->name;
+        if (backtracking(airports, target_path_len, path, path_len + 1))
+            return true;
+        to_airport->cnt++;
     }
+    return false;
+}
+
+char **findItinerary(char ***tickets, int ticketsSize, int *ticketsColSize,
+                     int *returnSize) {
+    from_airport_t *airports = NULL;
+
+    // 记录映射关系
     for (int i = 0; i < ticketsSize; i++) {
-        if ((used[i] == false) && (strcmp(start, tickets[i][0]) == 0)) {
-            result[*returnSize] = (char*)malloc(sizeof(char) * 4);
-            memcpy(result[*returnSize], tickets[i][1], sizeof(char) * 4);
-            (*returnSize)++;
-            used[i] = true;
-            /*if ((*returnSize) == ticketsSize + 1) {
-                return;
-            }*/
-            backtracting(tickets, ticketsSize, returnSize, tickets[i][1], result, used);
-            if (g_found) {
-                return;
-            }
-            (*returnSize)--;
-            used[i] = false;
+        from_airport_t *from_airport = NULL;
+        to_airport_t *to_airport = NULL;
+        HASH_FIND_STR(airports, tickets[i][0], from_airport);
+        if (!from_airport) {
+            from_airport = malloc(sizeof(from_airport_t));
+            from_airport->name = tickets[i][0];
+            from_airport->to_airports = NULL;
+            HASH_ADD_KEYPTR(hh, airports, from_airport->name,
+                            strlen(from_airport->name), from_airport);
+        }
+        HASH_FIND_STR(from_airport->to_airports, tickets[i][1], to_airport);
+        if (!to_airport) {
+            to_airport = malloc(sizeof(to_airport_t));
+            to_airport->name = tickets[i][1];
+            to_airport->cnt = 0;
+            HASH_ADD_KEYPTR(hh, from_airport->to_airports, to_airport->name,
+                            strlen(to_airport->name), to_airport);
         }
+        to_airport->cnt++;
     }
-    return;
-}
 
-char ** findItinerary(char *** tickets, int ticketsSize, int* ticketsColSize, int* returnSize){
-    if (tickets == NULL || ticketsSize <= 0) {
-        return NULL;
+    // 机场排序
+    for (from_airport *from_airport = airports; from_airport != NULL;
+         from_airport = from_airport->hh.next) {
+        HASH_SRT(hh, from_airport->to_airports, name_sort);
     }
-    result = malloc(sizeof(char*) * (ticketsSize + 1));
-    used = malloc(sizeof(bool) * ticketsSize);
-    memset(used, false, sizeof(bool) * ticketsSize);
-    result[0] = malloc(sizeof(char) * 4);
-    memcpy(result[0], "JFK", sizeof(char) * 4);
-    g_found = 0;
-    *returnSize = 1;
-    qsort(tickets, ticketsSize, sizeof(tickets[0]), cmp);
-    backtracting(tickets, ticketsSize, returnSize, "JFK", result, used);
+
+    char **path = malloc(sizeof(char *) * (ticketsSize + 1));
+    path[0] = "JFK";  // 起始机场
+    backtracking(airports, ticketsSize + 1, path, 1);
+
+    from_airport_destroy(airports);
+
     *returnSize = ticketsSize + 1;
-    return result;
+    return path;
 }
 ```