Solution of NumberOfDiscIntersections by Codility
question:https://codility.com/programmers/lessons/4
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 <algorithm> int solution(vector<int> &A) { // write your code in C++11 int size = A.size(); if (size <2) return 0; vector<long> begin; vector<long> end; for(int i=0; i<size; i++){ begin.push_back(i-static_cast<long>(A[i])); end.push_back(i+static_cast<long>(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<size; upper_index++){ while(lower_index<size && begin[lower_index]<= end[upper_index]) lower_index++; res += lower_index-upper_index-1; if (res >10000000) return -1; } return res; }