每日微软面试题——day 6（打印所有对称子串）

<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 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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 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 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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。

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