c++ substr函数

substr(字符串,截取开始位置,截取长度) //返回截取的字
substr(‘Hello World’,2,4) //返回结果为 ‘ello’
substr(‘Hello World’,-3,3)//返回结果为 ‘rld’ *负数(-i)表示截取的开始位置为字符串右端向左数第i个字符

#include<string>#include<iostream>using namespace std;main(){string s("12345asdf");string a=s.substr(0,5);       //获得字符串s中 从第0位开始的长度为5的字符串//默认时的长度为从开始位置到尾cout<<a<<endl;}输出结果为:12345

例AtCoder Beginner Contest 043
B - バイナリハックイージー / Unhappy Hacking (ABC Edit)
Time limit : 2sec / Memory limit : 256MB

Score : 200 points

Problem Statement
Sig has built his own keyboard. Designed for ultimate simplicity, this keyboard only has 3 keys on it: the 0 key, the 1 key and the backspace key.

To begin with, he is using a plain text editor with this keyboard. This editor always displays one string (possibly empty). Just after the editor is launched, this string is empty. When each key on the keyboard is pressed, the following changes occur to the string:

The 0 key: a letter 0 will be inserted to the right of the string.
The 1 key: a letter 1 will be inserted to the right of the string.
The backspace key: if the string is empty, nothing happens. Otherwise, the rightmost letter of the string is deleted.
Sig has launched the editor, and pressed these keys several times. You are given a string s, which is a record of his keystrokes in order. In this string, the letter 0 stands for the 0 key, the letter 1 stands for the 1 key and the letter B stands for the backspace key. What string is displayed in the editor now?

Constraints
1≦|s|≦10 (|s| denotes the length of s)
s consists of the letters 0, 1 and B.
The correct answer is not an empty string.
Input
The input is given from Standard Input in the following format:

s
Output
Print the string displayed in the editor in the end.

Sample Input 1
Copy
01B0
Sample Output 1
Copy
00
Each time the key is pressed, the string in the editor will change as follows: 0, 01, 0, 00.

Sample Input 2
Copy
0BB1
Sample Output 2
Copy
1
Each time the key is pressed, the string in the editor will change as follows: 0, (empty), (empty), 1.

题意：键盘上只有‘0’，‘1’‘B’,’B’代表删除键，输入一个有01B字符串，输出编辑器上显示的字符串。

#include <iostream>#include <cstdio>#include <string>using namespace std;int main(){    string s1,s2;    while(cin>>s1)    {        s2="";        for(int i=0;i<s1.size();i++)        {            if(s1[i]=='0')            {                s2+=s1[i];            }            if(s1[i]=='1')            {                s2+=s1[i];            }            if(s1[i]=='B')            {                s2=s2.substr(0,s2.size()-1);            }        }        cout<<s2<<endl;    }    return 0;}

另一种做法用vector

#include <iostream>#include <cstdio>#include <string>#include <vector>using namespace std;int main(){    string s;    vector<char> ch;    while(cin>>s)    {        int len1=s.size();        for(int i=0;i<len1;i++)        {            if(s[i]=='0')            {                ch.push_back('0');            }            else if(s[i]=='1')            {                ch.push_back('1');            }            else if(!ch.empty())            {                ch.pop_back();            }        }        int len2=ch.size();        for(int i=0;i<len2;i++)        {            cout<<ch[i];        }        cout<<endl;    }    return 0;}