01 分治算法

一、找到序列中第k小的数

#include<stdio.h>

void select_rank_k(int array[], int low, int high, int k, int* k_value){
    int left = low;
    int right = high;
    int pivot = array[low];
    int pivot_pos;
    while (left != right){
        while (array[right] >= pivot && left < right){
            right--;
        }
        array[left] = array[right];
        while (array[left] <= pivot && left < right){
            left++;
        }
        array[right] = array[left];
    }
    array[left] = pivot;
    pivot_pos = left;
    if (pivot_pos == k - 1) {
        *k_value = pivot;
        return;
    }else if (pivot_pos > k - 1){
        select_rank_k(array, low, pivot_pos - 1, k, k_value);
    }else{
        select_rank_k(array, pivot_pos + 1, high, k, k_value);
    }
}
int main(){
    // 输入n和k
    int n, k;
    scanf("%d %d", &n, &k);
    // 输入整数序列
    int nums[n];
    for (int i = 0; i < n; i++){
        scanf("%d", &nums[i]);
    }
    // 找到第k小数
    int k_value;
    select_rank_k(nums, 0, n - 1, k, &k_value);
    printf("%d", k_value);
    return 0;
}