Permutations

Signed-off-by: begeekmyfriend <[email protected]>

046_permutations/Makefile

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+all:
+	gcc -O2 -o test permutations.c

046_permutations/permutations.c

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+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+static void reverse(int *a, int size)
+{
+    int left = 0;
+    int right = size - 1;
+    while (left < right) {
+        int tmp = a[left];
+        a[left] = a[right];
+        a[right] = tmp;
+        left++;
+        right--;
+    }
+}
+
+static void next_permute(int* nums, int numsSize) {
+    if (numsSize <= 1) {
+        return;
+    }
+
+    int *p = nums + numsSize - 1;    
+    int *q = nums + numsSize - 1;
+
+    while (p != nums && *(p - 1) >= *p) {
+        p--;
+    }
+
+    if (p != nums) {
+        int n = *(p - 1);
+        while (*q <= n) {
+            q--;
+        }
+
+        int tmp = *q;
+        *q = *(p - 1);
+        *(p - 1) = tmp;
+    }
+    reverse(p, numsSize - (p - nums));
+}
+
+/**
+ ** Return an array of arrays of size *returnSize.
+ ** Note: The returned array must be malloced, assume caller calls free().
+ **/
+static int** permute(int* nums, int numsSize, int* returnSize) {
+    if (numsSize == 0) {
+        return NULL;
+    }
+
+    int i, count = 1, size = numsSize;
+    while (size > 0) {
+        count *= size--;
+    }
+
+    int **permutations = malloc(count * sizeof(int *));
+    permutations[0] = malloc(numsSize * sizeof(int));
+    memcpy(permutations[0], nums, numsSize * sizeof(int));
+    for (i = 1; i < count; i++) {
+        permutations[i] = malloc(numsSize * sizeof(int));
+        memcpy(permutations[i], permutations[i - 1], numsSize * sizeof(int));
+        next_permute(permutations[i], numsSize);
+    }
+
+    *returnSize = count;
+    return permutations;
+}
+
+int main(int argc, char **argv)
+{
+    if (argc <= 1) {
+        fprintf(stderr, "Usage: ./test ...\n");
+        exit(-1);
+    }
+
+    int i, j, count;
+    int *nums = malloc(sizeof(int));
+    for (i = 0; i < argc - 1; i++) {
+        nums[i] = atoi(argv[i + 1]);
+    }
+
+    int **lists = permute(nums, argc - 1, &count);
+
+    for (i = 0; i < count; i++) {
+        for (j = 0; j < argc - 1; j++) {
+            printf("%d", lists[i][j]);
+        }
+        putchar('\n');
+    }
+
+    return 0;
+}