Problem p03200 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
using namespace std;

#define lli long long
#define all(i) i.begin(), i.end()
#define rall(i) i.rbegin(), i.rend()
#define rep0(i, j) for (int i = 0; i < j; i++)
#define rep1(i, j) for (int i = 1; i <= j; i++)
#define rep0d(i, j) for (int i = j - 1; i >= 0; i--)
#define MOD 1000000007
#define INF (int)1e7
class Graph {
  typedef pair<int, int> p;

  int V;
  list<p> *adj;

public:
  Graph(int V) {
    this->V = V;
    adj = new list<p>[V];
  }

  void addEdge(int u, int v, int w) { adj[u].push_back(make_pair(v, w)); }

  bool route(lli from, lli to) { // vec[i]==j: j->iっていうもとに戻れる道がある
    vector<lli> vec(V, -1);
    dfs(1, from, vec);
    return vec[to] >= 0;
  }

  void dfs(lli ed, lli num, vector<lli> &vec) {
    for (auto e : adj[num]) {
      if (vec[e.first] != -1)
        continue;
      vec[e.first] = num;
      dfs(ed + 1, e.first, vec);
    }
  }

  vector<int> bfs(int n) {
    queue<int> q;
    vector<int> vec(V, -1);

    q.push(n);
    vec[n] = 0;
    while (!q.empty()) {
      int p = q.front();
      q.pop();
      for (auto e : adj[p]) {
        if (vec[e.first] == -1) {
          q.push(e.first);
          vec[e.first] = e.second + vec[p];
        }
      }
    }
    return vec;
  }

  vector<int> shortestPath(int src) {
    priority_queue<p, vector<p>, greater<p>> pq;

    vector<int> dist(V, INF);
    pq.push(make_pair(0, src));
    dist[src] = 0;

    while (!pq.empty()) {
      int u = pq.top().second;
      pq.pop();

      list<pair<int, int>>::iterator i;
      for (i = adj[u].begin(); i != adj[u].end(); ++i) {
        int v = (*i).first;
        int weight = (*i).second;

        if (dist[v] > dist[u] + weight) {
          dist[v] = dist[u] + weight;
          pq.push(make_pair(dist[v], v));
        }
      }
    }
    for (int i = 0; i < V; i++)
      if (dist[i] == INF)
        dist[i] = -1;
    return dist;
  }
};

bool cmpf(const pair<lli, lli> left, const pair<lli, lli> right) {
  return left.first < right.first;
}
bool cmps(const pair<lli, lli> left, const pair<lli, lli> right) {
  return left.second < right.second;
}

void dfs(lli ed, lli num, vector<map<lli, lli>> &vec) {
  lli n = 1;
  for (auto e : vec[num]) {
    if (n == ed)
      n++;
    if (e.second != -1)
      continue;
    vec[num][e.first] = n;
    vec[e.first][num] = n;
    dfs(n, e.first, vec);
    n++;
  }
}

lli gcd(lli a, lli b) {
  if (a < b)
    return gcd(b, a);
  lli temp;
  while ((temp = a % b)) {
    a = b;
    b = temp;
  }
  return b;
}

struct nCr {
  vector<lli> f, rf;
  lli count = 0;
  nCr(lli num) : f(num), rf(num) {
    count = num;
    f[0] = 1;
    rep1(i, num) f[i] = (f[i - 1] * i) % MOD;
    rep0(i, num + 1) rf[i] = inv(f[i]);
  }

  lli inv(lli x) {
    lli res = 1;
    lli k = MOD - 2;
    lli y = x;
    while (k) {
      if (k & 1)
        res = (res * y) % MOD;
      y = (y * y) % MOD;
      k /= 2;
    }
    return res;
  }
  lli C(lli n, lli k) {
    lli a = f[n];
    lli b = rf[n - k];
    lli c = rf[k];

    lli bc = (b * c) % MOD;

    return (a * bc) % MOD;
  }
};

struct UF {
  vector<int> par, size;

  UF(int N) : par(N), size(N, 1) { rep0(i, N) par[i] = i; }

  int root(int x) {
    if (par[x] == x)
      return x;
    return par[x] = root(par[x]);
  }

  void unite(int x, int y) {
    int rx = root(x);
    int ry = root(y);
    if (rx == ry)
      return;
    par[rx] = ry;
    size[rx] += size[ry];
  }

  bool same(int x, int y) {
    int rx = root(x);
    int ry = root(y);
    return rx == ry;
  }

  int getSize(int x) { return size[x]; }
};
int main() {
  string s;
  cin >> s;
  lli num = 0, ans = 0, wnum = 0;
  while (wnum != s.length() && num < s.length()) {
    while (s[num] == 'W' && num < s.length())
      num++;
    wnum = num;
    while (s[wnum] != 'W' && wnum < s.length())
      wnum++;
    if (!(wnum != s.length() && num < s.length()))
      break;
    ans += wnum - num;
    char c = s[wnum];
    s[wnum] = s[num];
    s[num] = c;
    num++;
  }
  cout << ans;
}


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

#define lli long long
#define all(i) i.begin(), i.end()
#define rall(i) i.rbegin(), i.rend()
#define rep0(i, j) for (int i = 0; i < j; i++)
#define rep1(i, j) for (int i = 1; i <= j; i++)
#define rep0d(i, j) for (int i = j - 1; i >= 0; i--)
#define MOD 1000000007
#define INF (int)1e7
class Graph {
  typedef pair<int, int> p;

  int V;
  list<p> *adj;

public:
  Graph(int V) {
    this->V = V;
    adj = new list<p>[V];
  }

  void addEdge(int u, int v, int w) { adj[u].push_back(make_pair(v, w)); }

  bool route(lli from, lli to) { // vec[i]==j: j->iっていうもとに戻れる道がある
    vector<lli> vec(V, -1);
    dfs(1, from, vec);
    return vec[to] >= 0;
  }

  void dfs(lli ed, lli num, vector<lli> &vec) {
    for (auto e : adj[num]) {
      if (vec[e.first] != -1)
        continue;
      vec[e.first] = num;
      dfs(ed + 1, e.first, vec);
    }
  }

  vector<int> bfs(int n) {
    queue<int> q;
    vector<int> vec(V, -1);

    q.push(n);
    vec[n] = 0;
    while (!q.empty()) {
      int p = q.front();
      q.pop();
      for (auto e : adj[p]) {
        if (vec[e.first] == -1) {
          q.push(e.first);
          vec[e.first] = e.second + vec[p];
        }
      }
    }
    return vec;
  }

  vector<int> shortestPath(int src) {
    priority_queue<p, vector<p>, greater<p>> pq;

    vector<int> dist(V, INF);
    pq.push(make_pair(0, src));
    dist[src] = 0;

    while (!pq.empty()) {
      int u = pq.top().second;
      pq.pop();

      list<pair<int, int>>::iterator i;
      for (i = adj[u].begin(); i != adj[u].end(); ++i) {
        int v = (*i).first;
        int weight = (*i).second;

        if (dist[v] > dist[u] + weight) {
          dist[v] = dist[u] + weight;
          pq.push(make_pair(dist[v], v));
        }
      }
    }
    for (int i = 0; i < V; i++)
      if (dist[i] == INF)
        dist[i] = -1;
    return dist;
  }
};

bool cmpf(const pair<lli, lli> left, const pair<lli, lli> right) {
  return left.first < right.first;
}
bool cmps(const pair<lli, lli> left, const pair<lli, lli> right) {
  return left.second < right.second;
}

void dfs(lli ed, lli num, vector<map<lli, lli>> &vec) {
  lli n = 1;
  for (auto e : vec[num]) {
    if (n == ed)
      n++;
    if (e.second != -1)
      continue;
    vec[num][e.first] = n;
    vec[e.first][num] = n;
    dfs(n, e.first, vec);
    n++;
  }
}

lli gcd(lli a, lli b) {
  if (a < b)
    return gcd(b, a);
  lli temp;
  while ((temp = a % b)) {
    a = b;
    b = temp;
  }
  return b;
}

struct nCr {
  vector<lli> f, rf;
  lli count = 0;
  nCr(lli num) : f(num), rf(num) {
    count = num;
    f[0] = 1;
    rep1(i, num) f[i] = (f[i - 1] * i) % MOD;
    rep0(i, num + 1) rf[i] = inv(f[i]);
  }

  lli inv(lli x) {
    lli res = 1;
    lli k = MOD - 2;
    lli y = x;
    while (k) {
      if (k & 1)
        res = (res * y) % MOD;
      y = (y * y) % MOD;
      k /= 2;
    }
    return res;
  }
  lli C(lli n, lli k) {
    lli a = f[n];
    lli b = rf[n - k];
    lli c = rf[k];

    lli bc = (b * c) % MOD;

    return (a * bc) % MOD;
  }
};

struct UF {
  vector<int> par, size;

  UF(int N) : par(N), size(N, 1) { rep0(i, N) par[i] = i; }

  int root(int x) {
    if (par[x] == x)
      return x;
    return par[x] = root(par[x]);
  }

  void unite(int x, int y) {
    int rx = root(x);
    int ry = root(y);
    if (rx == ry)
      return;
    par[rx] = ry;
    size[rx] += size[ry];
  }

  bool same(int x, int y) {
    int rx = root(x);
    int ry = root(y);
    return rx == ry;
  }

  int getSize(int x) { return size[x]; }
};
int main() {
  string s;
  cin >> s;
  lli num = 0, ans = 0, wnum = 0;
  while (wnum != s.length() && num < s.length()) {
    while (s[num] == 'W' && num < s.length())
      num++;
    if (num > wnum)
      wnum = num;
    while (s[wnum] != 'W' && wnum < s.length())
      wnum++;
    if (!(wnum != s.length() && num < s.length()))
      break;
    ans += wnum - num;
    char c = s[wnum];
    s[wnum] = s[num];
    s[num] = c;
    num++;
  }
  cout << ans;
}