Codeforces Round #547 (Div. 3) D. Colored Boots(思维)

#include <bits/stdc++.h>
#define ll long long
using namespace std;
struct Node{
  int x,y;
}aa[250005];
int n,m;

int main()
{
  scanf("%d",&n);
  string a,b;
  cin>>a>>b;
  map<char,vector<int>> ma1,ma2;
  for(int i=0;i<n;i++){
    ma1[a[i]].push_back(i + 1);
    ma2[b[i]].push_back(i + 1);
  }
  int cnt = 0;
  for(int i=0;i<26;i++){
    char ch = 'a' + i;
    while(ma1[ch].size() != 0 && ma2[ch].size() != 0){
      aa[cnt].x = ma1[ch][ma1[ch].size() - 1];
      aa[cnt++].y = ma2[ch][ma2[ch].size() - 1];
      ma1[ch].pop_back();
      ma2[ch].pop_back();
    }
  }
  char s = '?';
  while(!ma1[s].empty()){
    bool flag = false;
    for(int i=0;i<26;i++){
      char ch = 'a' + i;
      if(ma2[ch].size() != 0){
        flag = true;
        aa[cnt].x = ma1[s][ma1[s].size() - 1];
        aa[cnt++].y = ma2[ch][ma2[ch].size() - 1];
        ma1[s].pop_back();
        ma2[ch].pop_back();
      }
      if(ma1[s].size() == 0) break;
    }
    if(flag == false) break;
  }
  while(!ma2[s].empty()){
    bool flag = false;
    for(int i=0;i<26;i++){
      char ch = 'a' + i;
      if(ma1[ch].size() != 0){
        flag = true;
        aa[cnt].x = ma1[ch][ma1[ch].size() - 1];
        aa[cnt++].y = ma2[s][ma2[s].size() - 1];
        ma1[ch].pop_back();
        ma2[s].pop_back();
      }
      if(ma2[s].size() == 0) break;
    }
    if(flag == false) break;
  }
  while(ma2[s].size() != 0 && ma1[s].size() != 0){
    aa[cnt].x = ma1[s][ma1[s].size() - 1];
    aa[cnt++].y = ma2[s][ma2[s].size() - 1];
    ma1[s].pop_back();
    ma2[s].pop_back();
  }
  cout<<cnt<<endl;
  for(int i=0;i<cnt;i++){
    cout<<aa[i].x<<" "<<aa[i].y<<endl;
  }
  return 0;
}