1271B - Blocks - CodeForces Solution

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define pb push_back
#define pob pop_back()
#define vll vector<ll>
#define pll pair<ll,ll>
#define vpll vector<pll>
#define prq priority_queue<ll>
#define psq priority_queue<ll,vector<ll>,greater<ll>>
#define rep(i, a, b) for (ll i = a; i < b; i++)
#define repi(i, b, a) for (ll i = b; i >= a; i--)
#define input(x) {for(auto &i : x) cin>>i;}
#define input1(x) {rep(i,1,n+1) cin>>x[i];}
#define output(x) {for(auto &i : x) cout<<i<<" ";}
#define inf INT_MAX
#define minf INT_MIN
#define MOD 1000000007
#define all(x) x.begin(),x.end()
#define nline "\n"
#define yes cout<<"YES"<<nline;
#define no cout<<"NO"<<nline;


// #define priyanshu_2401 1


//--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
//Debug
#ifdef priyanshu_2401
#define debug(x) cerr << #x<<" "; cerr<<x<<" "; cerr << endl;
#else
#define debug(x);
#endif

// Operator overloads
template<typename T1, typename T2> // cin >> pair<T1, T2>
istream& operator>>(istream &istream, pair<T1, T2> &p) { return (istream >> p.first >> p.second); }
template<typename T> // cin >> vector<T>
istream& operator>>(istream &istream, vector<T> &v){for (auto &it : v)cin >> it;return istream;}
template<typename T1, typename T2> // cout << pair<T1, T2>
ostream& operator<<(ostream &ostream, const pair<T1, T2> &p) { return (ostream << p.first << " " << p.second); }
template<typename T> // cout << vector<T>
ostream& operator<<(ostream &ostream, const vector<T> &c) { for (auto &it : c) cout << it << " "; return ostream; }

// Mathematical functions
ll gcd(ll a, ll b){if (b == 0)return a;return gcd(b, a % b);} //__gcd 
ll lcm(ll a, ll b){return (a/gcd(a,b)*b);}
ll powermod(ll x, ll y, ll p){ll res = 1;x = x % p;if (x == 0) return 0;while (y > 0){if (y & 1)res = (res*x) % p;y = y>>1;x = (x*x) % p;}return res;}
ll modMultiply(ll x,ll y) {return ((x%MOD)*(y%MOD))%MOD;}
ll modAdd(ll x,ll y) {return ((x%MOD)+(y%MOD))%MOD;}
ll modSubtract(ll x,ll y) {return ((x%MOD)-(y%MOD)+MOD)%MOD;}
ll lengthOfNumber(ll x){ll count=0;while(x>0){count++;x/=10;}return count;}
 
vector<ll> fac(1002),inv(1002);
ll ncr(ll n,ll r){
    if(n<r) return 0;
    ll ans = modMultiply(modMultiply(fac[n],inv[r]),inv[n-r]);
    return ans;
}

//Graph-dfs
// bool gone[MN];
// vector<int> adj[MN];
// void dfs(int loc){
//     gone[loc]=true;
//     for(auto x:adj[loc])if(!gone[x])dfs(x);
// }

//Sorting
bool sorta(const pair<int,int> &a,const pair<int,int> &b){return (a.second < b.second);}
bool sortd(const pair<int,int> &a,const pair<int,int> &b){return (a.second > b.second);}

//Bits
string decToBinary(int n){string s="";int i = 0;while (n > 0) {s =to_string(n % 2)+s;n = n / 2;i++;}return s;}
ll binaryToDecimal(string n){string num = n;ll dec_value = 0;int base = 1;int len = num.length();for(int i = len - 1; i >= 0; i--){if (num[i] == '1')dec_value += base;base = base * 2;}return dec_value;}

//Check
bool isPrime(ll n){if(n<=1)return false;if(n<=3)return true;if(n%2==0||n%3==0)return false;for(int i=5;i*i<=n;i=i+6)if(n%i==0||n%(i+2)==0)return false;return true;}
bool isPowerOfTwo(int n){if(n==0)return false;return (ceil(log2(n)) == floor(log2(n)));}
bool isPerfectSquare(ll x){if (x >= 0) {ll sr = sqrt(x);return (sr * sr == x);}return false;}

//--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------


//code goes here

void solve()
{
	ll n;
	cin>>n;
	string s;
	cin>>s;
	
	ll b=0,w=0;

	for(auto &ch:s)
	{
		if(ch=='B') b++;
    	if(ch=='W') w++;
	}
	
	if(b&1 && w&1)
	{
		cout<<-1<<nline;
		return;
	}

	vll res;
	
	if(w&1)
	{
		ll i = 0;
		while(i<n)
		{
			if(s[i]=='B')
			{
				if(s[i+1]=='W')
				{
					res.pb(i+1);
					s[i+1] = 'B';
				}
				else
				{
					res.pb(i+1);
					s[i+1]='W';
				}
			}
			i++;
		}
	}
	else
	{
		ll i = 0;
		while(i<n)
		{
			if(s[i]=='W')
			{
				if(s[i+1]=='B')
				{
					res.pb(i+1);
					s[i+1]='W';
				}
				else
				{
					res.pb(i+1);
					s[i+1]='B';
				}
			}
			i++;
		}
	}

	cout<<res.size()<<nline;
	cout<<res<<nline;
}

int main() {
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);
    cout.tie(NULL);
	// ll t;
	// cin>>t;
	// while(t--)
	solve();
	return 0;
}