快速排序和快速选择

1.快速排序

这个快排的代码是基于算法导论中的介绍写的，将数组中最后一个元素作为pivot，设置两个指针i,j，都从数组头开始，当j遇到比pivot小的元素时，就将i和j的元素交换，否则j一直向后搜寻，i保持不动，最后将i递增1，与pivot元素交换，此时pivot前面的都是比pivot小的元素，后面都是比pivot大的元素，再分别对前后两个区间进行重复的操作。直到完全排序。

#include <iostream>#include <vector>using namespace std;int partition(vector<int> &nums, int start, int end) {int i = start - 1;for (int j = start; j < end; j++) {if (nums[j] <= nums[end]) {i = i + 1;int temp = nums[i];nums[i] = nums[j];nums[j] = temp;}}i = i + 1;int temp = nums[i];nums[i] = nums[end];nums[end] = temp;return i;}void quicksort(vector<int> &nums, int start, int end) {if (start < end) {int q = partition(nums, start, end);quicksort(nums, start, q - 1);quicksort(nums, q + 1, end);}}int main() {int a[7] = { 1,5,2,7,4,9,4 };vector<int> nums(a,a+7);int n = nums.size();quicksort(nums, 0, n - 1);for (int i = 0; i < n; i++) {cout << nums[i] << '\t';}}

2.快速选择

这个快速选择代码是找到数组中第k个元素（k从0到n-1），思想和上面的快排一致，只不过现在找到了第k个元素所在的区间后，只需在所在区间进行下一次搜寻，另一个区间就不用管了。

#include <iostream>#include <vector>using namespace std;int partition(vector<int> &nums, int start, int end) {int i = start - 1;for (int j = start; j < end; j++) {if (nums[j] <= nums[end]) {i = i + 1;int temp = nums[i];nums[i] = nums[j];nums[j] = temp;}}i = i + 1;int temp = nums[i];nums[i] = nums[end];nums[end] = temp;return i;}void quickselect(vector<int> &nums, int start, int end, int k) {if (start == end||start==k) {cout << nums[start];return;}if (end == k) {cout << nums[end];return;}if (start < end) {int q = partition(nums, start, end);if (q == k) {cout << nums[k];return;}else if (q > k) {quickselect(nums, start, q - 1, k);}else {quickselect(nums, q + 1, end, k);}}}int main() {int a[7] = { 1,5,2,7,4,9,4 };vector<int> nums(a, a + 7);int n = nums.size();quickselect(nums, 0, n - 1,1);}