<以下微软面试题全来自网络>
<以下答案与分析纯属个人观点，不足之处，还望不吝指出^_^>
<出处：http://blog.csdn.net/zhanxinhang>
题：1、如何判断一个字符串是对称的？如a，aa，aba。
2、如何利用2函数找出一个字符串中的所有对称子串？
分析：
且看第一个问题判断字符串对称，有以下两种方法。
方法一、使迭代器p1指向头，p2指向尾。使p1，p2相对而行(—> | <—)，每次比较p1，p2是否相等，直到它们相遇。
方法二、使p1、p2指向中间的一个元素（如果字符串长度为偶数的话就指向中间两个相邻的元素），使它们相向而行(<— |—>)，每次比较p1，p2是否相等。直到它们一个指向头一个指向尾便结束。
（可以看出方法1明显好于方法2）
现在看第二个问题打印所有的对称子串，判断对称子串的问题我们似乎已经解决，且有以上两种方法，针对现在的情况是否两种都合适呢，确切的说不是。如果是方法一，请想象一下，如果要p1，p2一个指向子串的头一个指向子串的尾，那么至少要两层循环一层控制p1一层控制p2，还有一层循环用于判断子串是否对称，也就是说总共需要三层嵌套循环，时间复杂度指数为3。而如果选择方法二呢？ 两层循环就能实现，不过要适应现在这种情况需要做些修改，就是针对偶数长度的子串进行一次遍历，再针对奇数长度的子串进行一次遍历，也就是两层嵌套循环中内层有两次循环，时间复杂度指数为2。详情且看代码。
实现加测试代码：
- /**
- Author:花心龟
- Blog：http://blog.csdn.net/zhanxinhang
- **/
- #include <iostream>
- #include <cstdlib>
- #include <ctime>
- using namespace std;
- class App
- {
- typedef string::iterator Iter_t;
- string m_str;
- void print(Iter_t p1, Iter_t p2) //打印从p1开始到p2结束的字符
- {
- while(p1<=p2)
- {
- cout<<*p1;
- p1++;
- }
- cout<<" | ";
- }
- bool isHuiwen1(Iter_t start,Iter_t end) //法1判断是否为对称子串
- {
- Iter_t p1 = start;
- Iter_t p2 = end;
- int times = (distance(p1,p2)+1) / 2; //比较次数
- while(times)
- {
- if(*p1 != *p2)
- return false;
- p1++;
- p2--;
- times--;
- }
- return true;
- }
- bool isHuiwen2(Iter_t mid_it) //法2判断是否为对称子串
- {
- Iter_t p1,p2;
- p1 = p2 = mid_it;
- while(p1>=m_str.begin() && p2 < m_str.end())
- {
- if(*p1 != *p2)
- break;
- print(p1,p2);//打印从p1到p2的字符
- p1--,p2++;
- }
- p1 = p2 = mid_it;
- p2++; //使p2向前移动一个位置，此时p1，p2分别指向中间两个相邻位置
- while(p1>=m_str.begin() && p2 < m_str.end())
- {
- if(*p1 != *p2)
- return false;
- print(p1,p2);//打印从p1到p2的字符
- p1--,p2++;
- }
- return true;
- }
- void show_all_substr_of_huiwen1() //法一打印所有对称子串
- {
- Iter_t p2=m_str.end()-1;
- Iter_t p1 = m_str.begin();
- for(;p1!=m_str.end();p1++)
- for(p2=p1;p2!=m_str.end();p2++)
- {
- if(isHuiwen1(p1,p2))
- print(p1,p2);
- }
- }
- void show_all_substr_of_huiwen2() //法二打印所有对称子串
- {
- Iter_t it = m_str.begin();
- for(;it!=m_str.end();it++)
- {
- isHuiwen2(it);
- }
- }
- public:
- void run()
- {
- m_str="abaaba";
- cout<<"测试数据为:abaaba"<<endl;
- show_all_substr_of_huiwen1();
- cout<<endl;
- show_all_substr_of_huiwen2();
- cout<<endl;
- m_str="asdfie";
- cout<<"测试数据为:asdfie"<<endl;
- show_all_substr_of_huiwen1();
- cout<<endl;
- show_all_substr_of_huiwen2();
- cout<<endl;
- m_str="aabaa";
- cout<<"测试数据为:aabaa"<<endl;
- show_all_substr_of_huiwen1();
- cout<<endl;
- show_all_substr_of_huiwen2();
- cout<<endl;
- //时间比较//
- m_str="this is a string for testing. aabaa alskjdfkljasdjflasdflkajsldkjfsjlakjsdlfjwoekjlakjlsdkjflsajlkdjfowieuoriuq aaddbb sldjfalkjsdlfkjasldjflajsldfjalskdjflakjsdlfkjaslkdjflajsdlfkjaslkdjflkajsdlkjflkasjdlfjaklsjdkfljaklsdjfklsajdflkjslkdjflaskjdlfkjalsdjlfkajsldfkjlaksjdfljasldjflaskjdfkasjdflaksjdkfljaskldfjlaksjdfljasldjflaksjdkljfkalsjdlkfjasldjflasjdlfjasldjfklsajdfljaskldfjlsakjdflkasjdfkl this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for 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for testing.make -k make -k make -k make -k make -k make -k is is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.make -k make -k make -k make -k make -k make -k is is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for testing.this is a string for 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- time_t start,record1;
- cout<<"show all substr of huiwen 1:";
- system("pause");
- start = time(NULL);
- show_all_substr_of_huiwen1();
- record1 = time(NULL);
- cout<<endl;
- cout<<"time:"<<record1-start;
- cout<<endl;
- cout<<"show all substr of huiwen 2:";
- system("pause");
- start = time(NULL);
- show_all_substr_of_huiwen2();
- record1 = time(NULL);
- cout<<endl;
- cout<<"time:"<<record1-start;
- cout<<endl;
- cout<<"(可以看到打印速度明显快于上一次)";
- }
- };
- int main()
- {
- App myapp;
- myapp.run();
- system("pause");
- return 0;
- }
测试结果：
各测试数据下的一二行为打印出来的所有对称字串。
按下任意键后：
以上是使用方法1打印出来的结果。
按下任意键后：
以上便是用方法2打印出来的结果。