例のアレ

12_5.cpp

#include <iostream>

using namespace std;

int main(int argc, char const *argv[]){
  int a[] = {10, 13, 13, 15, 20, 25};
  int b[] = {6, 6, 8, 8, 11, 13};
  int n = 6, w = 21;
  int i = n-1, j = 0;
  int i2 = i, j2 = j;
  int count = 0;
  while(i >= 0 && j < n){
    while(i2-1 >= 0 && a[i2-1] == a[i]) i2--;
    while(j2+1 < n && b[j] == b[j2+1]) j2++;
    int sum = a[i] + b[j];
    if(sum > w){
      i = --i2;
    }
    else if(sum < w){
      j = ++j2;
    }
    else {
      count += (i - i2 + 1) * (j2 - j + 1);
      i = --i2;
      j = ++j2;
    }
  }
  cout << count << endl;
  return 0;
}