Calculate the k-th number by using the Random Pseudo-fast sorting method

Divide and conquer the law to find the k-th number

The idea of "Quick Sort" is used to find the k-th large number of a series. The second year of high school started to write fast sort, but he has never been very clear about it. He knows that it is "divide", but does not understand the details. Today, with the help of this opportunity, I read the four fast-coding methods in programming Pearl River, which makes it clearer. This time, the quick ranking actually has a number. The number on the left is smaller than him, and the number on the right is bigger than him.

/*************************************** ********************************* // Randomize pseudo-fast sorting calculate the k-th number *//********************************* ***************************************/ # include <cstdio> # include <cstring> # include <algorithm> # include <cstdlib> using namespace STD; # define maxn 10100int num [maxn]; int qsort (int l, int R, int key) {int mid, LL = L, RR = R + 1; Mid = rand () % (R-l + 1) + L; swap (Num [L], num [Mi D]); While (LL <= RR) {do {ll ++;} while (LL <= R & num [ll] <num [l]); do {rr --;} while (Num [RR]> num [l]); If (LL <RR) {swap (Num [ll], num [RR]) ;}} swap (Num [L], num [RR]); If (RR-L + 1> = key) {If (RR-L + 1 = Key) return num [RR]; return qsort (L, RR, key);} else {return qsort (RR + 1, R, key-(RR-L + 1) ;}int main () {int N; while (~ Scanf ("% d", & N) {for (INT I = 0; I <n; ++ I) {scanf ("% d ", & num [I]);} int ans = qsort (0, n-1, n/2 + 1); printf ("% d \ n", ANS );} return 0 ;}