用C语言写该题目：给定一个可包含重复字母的序列，按任意顺序返回所有不重复的全排 列。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define MAX_N 100

void dfs(char *s, int len, int *used, char *path, int depth, char **res, int *resSize) {
    if (depth == len) {
        res[*resSize] = (char *)malloc(sizeof(char) * (len + 1));
        strcpy(res[*resSize], path);
        (*resSize)++;
        return;
    }

    for (int i = 0; i < len; i++) {
        if (used[i] || (i > 0 && s[i] == s[i - 1] && !used[i - 1])) {
            continue;
        }
        path[depth] = s[i];
        used[i] = 1;
        dfs(s, len, used, path, depth + 1, res, resSize);
        used[i] = 0;
    }
}

char **permuteUnique(char *s, int *returnSize) {
    int len = strlen(s);
    int used[MAX_N] = {0};
    char path[MAX_N] = {0};
    char **res = (char **)malloc(sizeof(char *) * MAX_N * MAX_N);
    *returnSize = 0;

    // 排序是为了剪枝
    qsort(s, len, sizeof(char), strcmp);
    dfs(s, len, used, path, 0, res, returnSize);

    return res;
}

int main() {
    char *s = "abc";
    int returnSize;

    char **res = permuteUnique(s, &returnSize);

    for (int i = 0; i < returnSize; i++) {
        printf("%s\n", res[i]);
    }

    return 0;
}