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Selectx3.java
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116 lines (106 loc) · 2.42 KB
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package Algorithm.selection;
import java.util.Arrays;
public class Selectx3 {
private static int compcount;
public static int select3(int[] arr, int n, int k) {
if (n==1){
return arr[0];
}
if (n < 25) {
insertionSort(arr, 0, n - 1);
return arr[k - 1];
} else {
return select(arr, 0, n - 1, k);
}
}
private static int select(int[] arr, int start, int end, int k) {
if (end <= start) {
return arr[start];
}
int v = findMedian(arr, start, end);
int lt = start;
int gt = end;
int i = start;
while (i <= gt) {
int comp = compare(v, arr[i]);
if (comp == 1) {
swap(arr, lt, i);
lt++;
i++;
} else if (comp == 0) {
i++;
} else {
swap(arr, i, gt);
gt--;
}
}
if (k <= lt - start) {
return select(arr, start, lt - 1, k);
} else if (k <= gt + 1 - start) {
return arr[lt];
} else {
return select(arr, gt + 1, end, k + start - gt - 1);
}
}
private static int findMedian(int[] arr, int start, int end) {
if (start == end || start + 1 == end) {
return arr[start];
}
int i = start;
int j = start;
while (i <= end) {
if (end - i == 3) {//do i need comparison here? to do docuemnt
insertionSort(arr, i, end);
swap(arr, i + 2, j);
} else if (end - i == 2) {
insertionSort(arr, i, end);
swap(arr, i + 1, j);
} else if (end - i == 1) {
insertionSort(arr, i, end);
swap(arr, i, j);
} else if (i == end) {
swap(arr, i, j);
} else {
insertionSort(arr, i, i + 4);
swap(arr, i + 2, j);
}
i = i + 5;
j++;
}
return findMedian(arr, start, start + (int) Math.ceil((end - start + 1) * 1.0 / 5 * 1.0));
}
public static void insertionSort(int[] arr, int l, int r) {
for (int i = l + 1; i < r + 1; i++) {
int j = i;
while (j > l && compare(arr[j], arr[j - 1]) == -1) {
swap(arr, j, j - 1);
j = j - 1;
}
}
}
public static int compare(int a, int b) {
compcount++;
if (a > b) {
return 1;
} else if (a == b) {
return 0;
} else {
return -1;
}
}
public static int getCount() {
return compcount;
}
public static void clearCount() {
compcount = 0;
}
private static void swap(int[] array, int i, int j) {
int temp = array[i];
array[i] = array[j];
array[j] = temp;
}
public static void main(String[] args) {
int [] arr=Util.createRandomArray(10000);
System.out.println(Selectx3.select3(arr,arr.length,arr.length/2)+"小雅的代码比较次数："+compcount);
}
}