question:

this question is seem like line intersections question. we can use similar method to solve this question. 

now i prove this solution. for each line, it has two endpoint. we call it left and right. assume that  the left and right have been sorted . 

if line i and line j intersection, we can conclude that left[i]<right[j] or left[j]<right[i]. ...  how to say?

?

 

 

trap: int overflow

code:

#include 
     
      int solution(vector
      
        &A) {    // write your code in C++11    int size = A.size();    if (size <2)        return 0;    vector
       
         begin;    vector
        
          end;    for(int i=0; i
         
          (A[i]));        end.push_back(i+static_cast
          
           (A[i])); } sort(begin.begin(),begin.end()); sort(end.begin(),end.end()); int res = 0; int upper_index =0, lower_index=0; for(upper_index =0; upper_index
           
            10000000) return -1; } return res;}