"Longest Consecutive Sequence"

Author: lennylxx

src/128.c

@@ -0,0 +1,80 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+struct HashNode {
+    int key;          /* num[i] */
+    int value;        /* i      */
+    struct HashNode *next;
+};
+
+static inline int hash(int key, int n) {
+    int index = key % n;
+    return (index > 0) ? (index) : (-index);
+}
+
+int calculate(struct HashNode **hashtable, int *length, int x, int n) {
+    int index = hash(x, n);
+    struct HashNode *p = hashtable[index];
+    while (p) {
+        if (p->key == x) { /* if found */
+            if (length[p->value] == 1) { /* x' length hasn't been calculated */
+                length[p->value] = calculate(hashtable, length, x + 1, n) + 1;
+            }
+            return length[p->value];
+        }
+        p = p->next;
+    }
+    return 0;/* not found */
+}
+
+int longestConsecutive(int num[], int n) {
+    int i;
+    struct HashNode **hashtable
+        = (struct HashNode **)calloc(n, sizeof(struct HashNode *));
+
+    for (i = 0; i < n; i++) {
+        struct HashNode *new_node
+            = (struct HashNode *)calloc(1, sizeof(struct HashNode));
+        new_node->key = num[i];
+        new_node->value = i;
+        new_node->next = NULL;
+        /* put new node into hash table */
+        int index = hash(num[i], n);
+        struct HashNode **p = hashtable + index;
+        /* get tail */
+        while (*p) {
+            p = &(*p)->next;
+        }
+        *p = new_node;
+    }
+
+    int *length = (int *)malloc(n * sizeof(int));
+    /* set all elements' length to 1 */
+    for (i = 0; i < n; i++){
+        length[i] = 1;
+    }
+
+    /* try to calculate the length of num[i] recursively */
+    for (i = 0; i < n; i++) {
+        if (length[i] > 1) continue; /* already calculated */
+        length[i] = calculate(hashtable, length, num[i] + 1, n) + 1;
+    }
+
+    int max_length = 1;
+    for (i = 0; i < n; i++){
+        if (length[i] > max_length)
+            max_length = length[i];
+    }
+    return max_length;
+}
+
+int main() {
+    int num1[] = { 100, 4, 200, 1, 3, 2 };
+    int num2[] = { 2, 3, 4, 5, 6, 7, 1 };
+    /* should be 4 */
+    printf("%d\n", longestConsecutive(num1, sizeof(num1) / sizeof(num1[0])));
+    /* should be 7 */
+    printf("%d\n", longestConsecutive(num2, sizeof(num2) / sizeof(num2[0])));
+    return 0;
+}