Codeforces Round #608 (Div. 2) B. Blocks

链接：

https://codeforces.com/contest/1271/problem/B

题意：

There are n blocks arranged in a row and numbered from left to right, starting from one. Each block is either black or white.

You may perform the following operation zero or more times: choose two adjacent blocks and invert their colors (white block becomes black, and vice versa).

You want to find a sequence of operations, such that they make all the blocks having the same color. You don't have to minimize the number of operations, but it should not exceed 3⋅n. If it is impossible to find such a sequence of operations, you need to report it.

思路：

只要有偶数颜色就能修改完，
把偶数修改为奇数即可，都为偶数随便。

代码：

#include<bits/stdc++.h>
using namespace std;

char s[210];
int n;

int main()
{
    cin >> n >> s;
    int b = 0, w = 0;
    for (int i = 0;i < n;i++)
    {
        if (s[i] == 'B')
            b++;
        else
            w++;
    }
    if (b&1 && w&1)
    {
        puts("-1");
        return 0;
    }
    if (b == 0 || w == 0)
    {
        puts("0");
        return 0;
    }
    vector<int> ans;
    int op = b&1 ? 1 : 2;
    for (int i = n-1;i > 0;i--)
    {
        if (op == 1 && s[i] == 'W')
            s[i] = 'B', s[i-1] = (s[i-1] == 'B' ? 'W' : 'B'), ans.push_back(i);
        else if (op == 2 && s[i] == 'B')
            s[i] = 'W', s[i-1] = (s[i-1] == 'W' ? 'B' : 'W'), ans.push_back(i);
    }
    cout << (int)ans.size() << endl;
    for (auto v: ans)
        cout << v << ' ' ;
    cout << endl;

    return 0;
}