Problem p02597 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
using namespace std;
#define pb push_back
#define INF 1000000000
#define mod 1000000007
#define vi vector<int>
#define vll vector<long long>
#define ii pair<int, int>
#define vii vector<pair<int, int>>
#define ll long long
#define ld long double
#define all(x) x.begin(), x.end()
#define ff first
#define ss second
#define lb lower_bound
#define ub upper_bound
#define sz(a) (int)a.size()

long long binpow(long long a, long long b, long long m) {
  a %= m;
  long long res = 1;
  while (b > 0) {
    if (b & 1)
      res = res * a % m;
    a = a * a % m;
    b >>= 1;
  }
  return res;
}
/*struct item{
    int m,c;
};					// change item according to the
question; struct segtree{ int size; vector<item>values; void intit(int n){
                size=1;
                while(size<n)size*=2;
                values.resize(2*size);
        }
    item merge(item a,item b){
        if(a.m<b.m)return a;
        if(b.m<a.m)return b;
        if(a.m==b.m)return {a.m,a.c+b.c};
    }
    item single(int v){
        return {v,1};
    }
    item NEUTRAL_ELEMENT={INF,1};
    void build(vi &a,int x,int lx,int rx){
        if(rx-lx==1){
            if(lx<sz(a)){
                values[x]=single(a[lx]);
            }
            return;
        }
        int m=(lx+rx)/2;
        build(a,2*x+1,lx,m);
        build(a,2*x+2,m,rx);
        values[x]=merge(values[2*x+1],values[2*x+2]);
    }
    void build(vi &a){
        build(a,0,0,size);
    }
    void set(int i,int v,int x,int lx,int rx){
        if(rx-lx==1){
            values[x]=single(v);
            return;
        }
        int m=(lx+rx)/2;
        if(i<m){
            set(i,v,2*x+1,lx,m);
        }else{
            set(i,v,2*x+2,m,rx);
        }
        values[x]=merge(values[2*x+1],values[2*x+2]);
    }
    void set(int i,int v){
        set(i,v,0,0,size);
    }
    item calc(int l,int r,int x,int lx,int rx){
        if(lx>=r||rx<=l)return NEUTRAL_ELEMENT;
        if(l<=lx && rx<=r)return values[x];
        int m=(lx+rx)/2;
        item s1=calc(l,r,2*x+1,lx,m);
        item s2=calc(l,r,2*x+2,m,rx);
        return merge(s1,s2);
    }
    item calc(int l,int r){
        return calc(l,r,0,0,size);
    }
};*/
int main() {
  int n;
  cin >> n;
  string s;
  cin >> s;
  vi idxr, idxw;
  for (int i = 0; i < n; i++) {
    if (s[i] == 'R')
      idxr.pb(i);
    else
      idxw.pb(i);
  }
  if (sz(idxr) == 0 || sz(idxw) == 0) {
    cout << 0;
    return 0;
  }
  int cnt = 0;
  for (int i = 0; i < sz(idxr); i++) {
    if (idxr[sz(idxr) - 1] > idxw[0]) {
      int temp = idxr[sz(idxr) - 1];
      idxr[sz(idxr) - 1] = idxw[0];
      idxw[0] = temp;
      cnt++;
      sort(all(idxr));
      sort(all(idxw));
    }
  }
  cout << cnt;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
#define pb push_back
#define INF 1000000000
#define mod 1000000007
#define vi vector<int>
#define vll vector<long long>
#define ii pair<int, int>
#define vii vector<pair<int, int>>
#define ll long long
#define ld long double
#define all(x) x.begin(), x.end()
#define ff first
#define ss second
#define lb lower_bound
#define ub upper_bound
#define sz(a) (int)a.size()

long long binpow(long long a, long long b, long long m) {
  a %= m;
  long long res = 1;
  while (b > 0) {
    if (b & 1)
      res = res * a % m;
    a = a * a % m;
    b >>= 1;
  }
  return res;
}
/*struct item{
    int m,c;
};					// change item according to the
question; struct segtree{ int size; vector<item>values; void intit(int n){
                size=1;
                while(size<n)size*=2;
                values.resize(2*size);
        }
    item merge(item a,item b){
        if(a.m<b.m)return a;
        if(b.m<a.m)return b;
        if(a.m==b.m)return {a.m,a.c+b.c};
    }
    item single(int v){
        return {v,1};
    }
    item NEUTRAL_ELEMENT={INF,1};
    void build(vi &a,int x,int lx,int rx){
        if(rx-lx==1){
            if(lx<sz(a)){
                values[x]=single(a[lx]);
            }
            return;
        }
        int m=(lx+rx)/2;
        build(a,2*x+1,lx,m);
        build(a,2*x+2,m,rx);
        values[x]=merge(values[2*x+1],values[2*x+2]);
    }
    void build(vi &a){
        build(a,0,0,size);
    }
    void set(int i,int v,int x,int lx,int rx){
        if(rx-lx==1){
            values[x]=single(v);
            return;
        }
        int m=(lx+rx)/2;
        if(i<m){
            set(i,v,2*x+1,lx,m);
        }else{
            set(i,v,2*x+2,m,rx);
        }
        values[x]=merge(values[2*x+1],values[2*x+2]);
    }
    void set(int i,int v){
        set(i,v,0,0,size);
    }
    item calc(int l,int r,int x,int lx,int rx){
        if(lx>=r||rx<=l)return NEUTRAL_ELEMENT;
        if(l<=lx && rx<=r)return values[x];
        int m=(lx+rx)/2;
        item s1=calc(l,r,2*x+1,lx,m);
        item s2=calc(l,r,2*x+2,m,rx);
        return merge(s1,s2);
    }
    item calc(int l,int r){
        return calc(l,r,0,0,size);
    }
};*/
int main() {
  int n;
  cin >> n;
  string s;
  cin >> s;
  vi idxr, idxw;
  for (int i = 0; i < n; i++) {
    if (s[i] == 'R')
      idxr.pb(i);
    else
      idxw.pb(i);
  }
  if (sz(idxr) == 0 || sz(idxw) == 0) {
    cout << 0;
    return 0;
  }
  int cnt = 0, pos = sz(idxr) - 1;
  while (idxr[pos] > idxw[sz(idxr) - pos - 1] && pos >= 0) {
    cnt++, pos--;
  }
  cout << cnt;
}