Educational Codeforces Round 30-B-Balanced Substring

B-Balanced Substring
题目链接：http://codeforces.com/contest/873/problem/B

Description

You are given a string s consisting only of characters 0 and 1. A substring [l, r] of s is a string slsl + 1sl + 2... sr, and its length equals to r - l + 1. A substring is called balanced if the number
of zeroes (0) equals to the number of ones in this substring.

You have to determine the length of the longest balanced substring of s.

Input
The first line contains n (1 ≤ n ≤ 100000) — the number of characters in s.

The second line contains a string s consisting of exactly n characters. Only characters 0 and 1 can appear in s.

Output
If there is no non-empty balanced substring in s, print 0. Otherwise, print the length of the longest balanced substring.

Examples
input
8
11010111
output
4
input
3
111
output
0
Note
In the first example you can choose the substring [3, 6]. It is balanced, and its length is 4. Choosing the substring [2, 5] is also possible.

In the second example it's impossible to find a non-empty balanced substring.

题意：给定一个01串，找到最长的01个数相同的子串。
题解：将0赋值为-1,1赋值为1.求维护每个长度的前缀和，有相同的两个前缀和时说明他们之间的01个数相等。注意 
  不能用数组维护，会出现-1越界的情况。
AC代码：

/*
* @Author: 王文宇
* @Date:   2017-10-19 02:37:52
* @Last Modified by:   王文宇
* @Last Modified time: 2017-10-19 22:27:48
*/
#include <iostream>
#include <algorithm>
#include <map>
using namespace std;
const int maxn = 100007;
char s[maxn];
int a[maxn];
map<int,int>k;
int main()
{
int n;
cin>>n>>s;
int c=0;
for(int i=0;i<n;i++)
{
if(s[i]=='0')
{
c++;
a[i]=-1;
}
else a[i]=1;
}
int sum = 0;
int ans = 0;
k[0]=1;
for(int i=0;i<n;i++)
{
ans +=a[i];
if(k[ans])
{
sum=max(sum,i-k[ans]+1);
}
else k[ans]=i+1;
}
cout<<sum<<endl;
return 0;
}