选择排序

1. 基础版

每次从剩余待排元素中找到最小值放到待排元素最前面，共n-1轮

import java.util.*;
public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] a = new int[n];
        for (int i = 0; i < n; i++)
            a[i] = sc.nextInt();
        for (int i = 0; i < a.length - 1; i++) {
            for (int j = i + 1; j < a.length; j++) {
                if (a[j] < a[i]) {
                    int t = a[i];
                    a[i] = a[j];
                    a[j] = t;
                }
            }
        }
        for (int i = 0; i < n; i++) {
            System.out.print(a[i] + " ");
        }
    }
}

2. 基础优化版

不是在选数过程中进行交换，而是找到最小元素后再交换

import java.util.*;
public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] a = new int[n];
        for (int i = 0; i < n; i++)
            a[i] = sc.nextInt();
        for (int i = 0; i < a.length - 1; i++) {
            int mIdx = i;
            for (int j = i + 1; j < a.length; j++) {
                if (a[j] < a[mIdx])
                    mIdx = j;
            }
            if (mIdx != i) {
                int t = a[i];
                a[i] = a[mIdx];
                a[mIdx] = t;
            }
        }
        for (int i = 0; i < n; i++) {
            System.out.print(a[i] + " ");
        }
    }
}

复杂度

时间复杂度：严格O(n)
空间复杂度：O(1)