【百度面试题】求包括固定字符集的最短子串
问题描写叙述:
一串首尾相连的珠子(m个)。有N种颜色(N<=10),设计一个算法。取出当中一段,要求包括全部N中颜色,并使长度最短。
并分析时间复杂度与空间复杂度。
问题可等同于求一个长字符串c中包括固定字符集target的最短子串
如"abddcbda"中包括"abc"的最短子串是"cbda"
算法:
指针head,rear分别指向眼下已知的最短子串。初始值为c的头和尾
指针begain。end是当前检查的子串,一旦该串包括了所有target,就比較它和已知最短子串的长度。若它更短,设为新的最短子串
数组targetlist记录target中每一个元素在begain-end段中出现的次数
begain end初始值均为c的头,对begain end进行的操作例如以下:
①若end在target中,targetlist中对应元素(end出现的次数)加一,转②若end不在target中转④
②从begain開始检查。若begain出现的次数大于1或者begain不在target中。begain前移。继续检查。直到begain仅仅出现一次。
这一步保证begain-end段是在不丢失target的情况下最短的转③
③检车begain-end段,一旦该段包括了所有target,就比較它和已知最短子串(head-rear段)的长度,若它更短。设为新的最短子串。begain++,转④
④end++转①
这样仅仅需一次遍历。就能够找出最短子串,时间复杂度为O(n),n为c长度
空间复杂度为O(n+m) ,m为字符集target中元素的个数
代码实现:
#pragma once #include<iostream> using namespace std; bool Intarget(char* end, char* target, int* clist, int* (&targetlist), int count) { for (int i = 0; i < (int)strlen(target); i++) { if (*(target + i) == *end) { targetlist[i]++;//出现此事加1 clist[count] = i;//end指向的元素是target中第i个元素 return true; } } return false; } //缩减begain-end段,保证它里面目标个数不变的情况下,该段最小 void Reduce(char*(&c), char*(&target),char*(&begain), char*(&end), int*(&clist), int* (&targetlist)) { for (int i = begain - c; i <= end - c; i++) { if (targetlist[clist[i]] > 1)//begain指向的元素出现次数大于1,,begain前移 { begain++; targetlist[clist[i]]--; } else if (clist[i] == -1)//begain指向的元素不在target中,begain前移 begain++; else return; } } //检查begain-end是否包括全部target,若是。且它长度小于head-rear段,将它设为新的head-rear void Checkall(char*(&c), char*(&target), int*targetlist, int* clist, char*(&begain), char*(&end), char*(&head), char*(&rear)) { bool all = true; //检查begain-end这一段,target中的元素是否全都出现 for (int i = 0; i < (int)strlen(target); i++) if (targetlist[i]>0) continue; else { all = false; break; } //head-rear取小值 if (all&& end - begain < rear - head) { head = begain; rear = end; targetlist[clist[begain - c]]--; begain++; } } void Find(char* c, char* target, char* (&head), char* (&rear)) { int *targetlist = new int[strlen(target)]; for (int i = 0; i < (int)strlen(target); i++)//target中元素已出现的次数 targetlist[i] = 0; int *clist = new int[strlen(c)]; for (int i = 0; i < (int)strlen(c); i++)//c中第i个元素在target中的位置 clist[i] = -1; char *begain, *end;//当前正在检查的段 head = c;//指向头 rear = head + strlen(c)-1;//指向尾 begain = end = c; while (*end) { if (Intarget(end, target, clist, targetlist, end - c))//假设*end在target中 { //缩减begain-end段 Reduce(c, target, begain, end, clist, targetlist); //检查begain-end段是否包括全部target。是,则比較长度,小了就生成新的head。rear Checkall(c, target, targetlist,clist, begain, end, head, rear); } end++; } } void main() { char* c = "abddcbdabcd"; char* target = "abc"; int *targetlist = new int[strlen(target)]; char *head = new char; char* rear = new char;//保存最小段 Find(c, target, head, rear); cout << "最短子串长度:" << rear - head +1<< endl; cout << "最短子串:"; for (int i = 0; i <= rear - head; i++) cout << *(head + i)<<' '; cout << endl; cout << "最短子串的head位置:" << head - c << endl; cout << "最短子串的rear位置:" << rear - c << endl; system("pause"); }
执行结果: