字符串长度:n
字符可能个数:m(剑指OFFER中m为256)
剑指OFFER P187中的方式平均约循环次数为:1*n ~ 2*n(依赖于出现一次的字符所处的位置),耗时也跟着浮动:1*t~2*t
原本的思路为
1、读取每一个字符,对每个字符进行计数(循环n次)
2、再次读取每一个字符,直到遇到计数为1的字符为止
优化第二点:采用一个数组记录下每个字符第一次出现的位置,计数大于1的设为最大值(字符串最大长度),对其进行排序(mlogm),第一个就是第一次出现的。
优化后的好处一是时间稳定在1.5*t,二是不需要再次读取输入字符串,节省内存。(注:下面的代码并未完善,合适的方式应该是读取一个字符便处理一次,因而额外的空间为3*m*24;旧的方式需要的额外空间为1*m*24+n.)
另外,0x21~0x7f这个范围设定是360笔试中的^_^
#include <stdio.h> #include <string.h> #include <stdlib.h> #include <time.h> //0x7F - 0x21 = 94 // 0,1,2... 94 分别记录 0x21,0x22,...,0x7F #define MAX_STRING_LENGTH 100000 #define MAX_CHAR_NUMBER 95 typedef struct __CountCharInfo { char orgidx; long long count; long long seq; } CountCharInfo; int ccicomp(const void*a,const void*b) { CountCharInfo *a1 = (CountCharInfo*)a; CountCharInfo *b1 = (CountCharInfo*)b; return (a1->seq - b1->seq); } void cciinit(CountCharInfo *cci, int n) { for (int i = 0; i < n; i++) { cci[i].orgidx= i; cci[i].count = 0; cci[i].seq = MAX_STRING_LENGTH; } } char GetOnetimeChar(char s[]) { CountCharInfo cci[MAX_CHAR_NUMBER]; cciinit(cci, MAX_CHAR_NUMBER); long long len = strlen(s); long long i; char j; bool exist = 0; for (i = 0; i < len; i++) { cci[s[i] - 0x21].count++; if (cci[s[i] - 0x21].seq == MAX_STRING_LENGTH) { cci[s[i] - 0x21].seq = i; } } for (j = 0; j < MAX_CHAR_NUMBER; j++) { if (cci[j].count != 1) { cci[j].seq = MAX_STRING_LENGTH; } else { exist = 1; } } //cci.seq排序 if (exist) { qsort(cci, MAX_CHAR_NUMBER, sizeof(cci[0]), ccicomp); } else { return 0; } return (0x21 + cci[0].orgidx); } char GetOnetimeCharOld(char s[]) { CountCharInfo cci[MAX_CHAR_NUMBER]; cciinit(cci, MAX_CHAR_NUMBER); long long len = strlen(s); long long i; bool exist = 0; for (i = 0; i < len; i++) { cci[s[i] - 0x21].count++; } for (i = 0; i < len; i++) { if (cci[s[i] - 0x21].count == 1) { return (0x21 + cci[s[i] - 0x21].orgidx); } } return 0; } void GenerateString(char *str, int len) { srand(time(NULL)); for (long long i = 0; i < len; i++) { str[i] = rand()%(0x7E - 0x21) + 0x21; } str[len-2] = 0x7F; str[len-1] = '\0'; } int main() { char str[MAX_STRING_LENGTH] = ""; char out; int i, n; char strTest[MAX_STRING_LENGTH] = ""; clock_t start, end; GenerateString(strTest, MAX_STRING_LENGTH); start = clock(); for(i = 0; i < 100; i++) { out = GetOnetimeChar(strTest); } end = clock(); printf("\t----%d-%f----\n", out, (double)(start-end)/CLOCKS_PER_SEC); start = clock(); for(i = 0; i < 100; i++) { out = GetOnetimeCharOld(strTest); } end = clock(); printf("\t----%d-%f----\n", out, (double)(start-end)/CLOCKS_PER_SEC); /* scanf("%d\n", &n); for (i = 0; i < n; i++) { memset(str, 0, 1000000); scanf("%s", str); out = GetOnetimeChar(str); printf("%c\n", out); } */ getchar(); }