Codeforces Round #430 (Div. 2)

题目链接：http://codeforces.com/contest/842/problem/B

题意：给定一个圆心在原点(0,0)半径为r的大圆和一个圆内的圆环长度d，然后给你n个小圆，问你有多少个小圆完全位于圆环中

思路：求出每个小圆的圆心和原点的距离dist。 如果满足dist[i]-r[i]>=(r-d)&&dist[i]+r[i]<=r,说明这个小圆完全位于圆环。

#define _CRT_SECURE_NO_DEPRECATE
#include<iostream>
#include<cstring>
#include<string>
#include<algorithm>
#include<stdio.h>
#include<queue>
#include<vector>
#include<stack>
#include<map>
#include<set>
#include<time.h>
#include<cmath>
#include<sstream>
#include<assert.h>
using namespace std;
typedef long long int LL;
const int inf = 0x3f3f3f3f;
const LL INF = 0x3f3f3f3f3f3f3f3fLL;
const int MAXN = 1e5 + 24;
int main(){
//#ifdef kirito
//    freopen("in.txt", "r", stdin);
//    freopen("out.txt", "w", stdout);
//#endif
//    int start = clock();
    int R,d,n,x,y,r;
    while (~scanf("%d%d",&R,&d)){
        scanf("%d", &n); int ans = 0;
        for (int i = 1; i <= n; i++){
            scanf("%d%d%d", &x, &y, &r);
            double dist = sqrt(1.0*x*x + 1.0*y*y);
            if ((dist-r) >= (R - d) && (dist+r) <= R){
                ans++;
            }
        }
        printf("%d
", ans);
    }
//#ifdef LOCAL_TIME
//    cout << "[Finished in " << clock() - start << " ms]" << endl;
//#endif
    return 0;