题目：

题解：

#define MAX_STR_LEN 32
typedef unsigned long long ULL;void reverseStr(char * str) {int n = strlen(str);for (int l = 0, r = n-1; l < r; l++, r--) {char c = str[l];str[l] = str[r];str[r] = c;}
}ULL * getCandidates(const char * n, int * returnSize) {int len = strlen(n);int pos = 0;ULL * candidates = (ULL *)malloc(sizeof(ULL) * 5);candidates[pos++] = (ULL)pow(10, len) + 1;candidates[pos++] = (ULL)pow(10, len - 1) - 1;char str[MAX_STR_LEN], prefix[MAX_STR_LEN];char candidate[MAX_STR_LEN];snprintf(str, (len + 1) / 2 + 1, "%s", n);ULL selfPrefix = atol(str);    for (ULL i = selfPrefix - 1; i <= selfPrefix + 1; i++) {sprintf(prefix, "%ld", i);sprintf(candidate, "%s", prefix);reverseStr(prefix);sprintf(candidate + strlen(candidate), "%s", prefix + (len & 1));candidates[pos++] = atoll(candidate);}*returnSize = pos;return candidates;
}char * nearestPalindromic(char * n){ULL selfNumber = atoll(n), ans = -1;int candidatesSize = 0;const ULL * candidates = getCandidates(n, &candidatesSize);for (int i = 0; i < candidatesSize; i++) {if (candidates[i] != selfNumber) {if (ans == -1 ||labs(candidates[i] - selfNumber) < labs(ans - selfNumber) ||labs(candidates[i] - selfNumber) == labs(ans - selfNumber) && candidates[i] < ans) {ans = candidates[i];}}}char * str = (char *)malloc(sizeof(char) * MAX_STR_LEN);sprintf(str, "%ld", ans);free(candidates);return str;
}