题目链接：http://www.51nod.com/onlineJudge/questionCode.html#!problemId=1105
先排序，二分上下界分别是最小的两个数和最大的两个数的乘积。注意到一个性质，就是a[i]*b[j]，i从左到右，j从右到左。假如遇到一组a[i]*b[j]<=x，j就不必再减小了。这时候比x小的数一定是j个。下一次继续找i，这个b[j]一定不可能大于b[j+1]。问题就由找第k大的数，转换成找第n*n-k+1小的数。每次二分这个乘积x，看看是否符合条件即可。
1 /* 2 ━━━━━┒ギリギリ♂ eye！ 3 ┓┏┓┏┓┃キリキリ♂ mind！ 4 ┛┗┛┗┛┃＼○／ 5 ┓┏┓┏┓┃ / 6 ┛┗┛┗┛┃ノ) 7 ┓┏┓┏┓┃ 8 ┛┗┛┗┛┃ 9 ┓┏┓┏┓┃ 10 ┛┗┛┗┛┃ 11 ┓┏┓┏┓┃ 12 ┛┗┛┗┛┃ 13 ┓┏┓┏┓┃ 14 ┃┃┃┃┃┃ 15 ┻┻┻┻┻┻ 16 */ 17 #include <algorithm> 18 #include <iostream> 19 #include <iomanip> 20 #include <cstring> 21 #include <climits> 22 #include <complex> 23 #include <fstream> 24 #include <cassert> 25 #include <cstdio> 26 #include <bitset> 27 #include <vector> 28 #include <deque> 29 #include <queue> 30 #include <stack> 31 #include <ctime> 32 #include <set> 33 #include <map> 34 #include <cmath> 35 using namespace std; 36 #define fr first 37 #define sc second 38 #define cl clear 39 #define BUG puts("here!!!") 40 #define W(a) while(a--) 41 #define pb(a) push_back(a) 42 #define Rint(a) scanf("%d", &a) 43 #define Rll(a) scanf("%lld", &a) 44 #define Rs(a) scanf("%s", a) 45 #define Cin(a) cin >> a 46 #define FRead() freopen("in", "r", stdin) 47 #define FWrite() freopen("out", "w", stdout) 48 #define Rep(i, len) for(int i = 0; i < (len); i++) 49 #define For(i, a, len) for(int i = (a); i < (len); i++) 50 #define Cls(a) memset((a), 0, sizeof(a)) 51 #define Clr(a, x) memset((a), (x), sizeof(a)) 52 #define Full(a) memset((a), 0x7f7f, sizeof(a)) 53 #define lp p << 1 54 #define rp p << 1 | 1 55 #define pi 3.14159265359 56 #define RT return 57 #define lowbit(x) x & (-x) 58 #define onenum(x) __builtin_popcount(x) 59 typedef long long LL; 60 typedef long double LD; 61 typedef unsigned long long ULL; 62 typedef pair<int, int> pii; 63 typedef pair<string, int> psi; 64 typedef map<string, int> msi; 65 typedef vector<int> vi; 66 typedef vector<LL> vl; 67 typedef vector<vl> vvl; 68 typedef vector<bool> vb; 69 70 const int maxn = 50050; 71 LL n, k, ans; 72 LL a[maxn], b[maxn]; 73 74 bool ok(LL x) { 75 LL ret = 0; 76 LL j = n; 77 For(i, 1, n+1) { 78 while(j) { 79 if(a[i] * b[j] > x) j--; 80 else break; 81 } 82 ret += j; 83 } 84 return ret >= k; 85 } 86 87 int main() { 88 // FRead(); 89 while(cin >> n >> k) { 90 k = n * n - k + 1; 91 For(i, 1, n+1) { 92 cin >> a[i] >> b[i]; 93 } 94 sort(a+1, a+n+1); sort(b+1, b+n+1); 95 LL l = a[1] * b[1], r = a[n] * b[n]; 96 while(l <= r) { 97 LL m = (l + r) >> 1; 98 if(ok(m)) { 99 r = m - 1; 100 ans = m; 101 } 102 else l = m + 1; 103 } 104 cout << ans << endl; 105 } 106 RT 0; 107 }