130 lines
3.6 KiB
Java
130 lines
3.6 KiB
Java
package Others;
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import java.util.Arrays;
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/**
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* BFPRT algorithm.
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*/
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public class BFPRT {
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public static int[] getMinKNumsByBFPRT(int[] arr, int k) {
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if (k < 1 || k > arr.length) {
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return null;
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}
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int minKth = getMinKthByBFPRT(arr, k);
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int[] res = new int[k];
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int index = 0;
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for (int i = 0; i < arr.length; i++) {
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if (arr[i] < minKth) {
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res[index++] = arr[i];
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}
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}
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for (; index != res.length; index++) {
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res[index] = minKth;
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}
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return res;
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}
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public static int getMinKthByBFPRT(int[] arr, int k) {
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int[] copyArr = copyArray(arr);
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return bfprt(copyArr, 0, copyArr.length - 1, k - 1);
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}
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public static int[] copyArray(int[]arr) {
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int[] copyArr = new int[arr.length];
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for(int i = 0; i < arr.length; i++) {
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copyArr[i] = arr[i];
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}
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return copyArr;
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}
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public static int bfprt(int[] arr, int begin, int end, int i) {
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if (begin == end) {
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return arr[begin];
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}
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int pivot = medianOfMedians(arr, begin, end);
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int[] pivotRange = partition(arr, begin, end, pivot);
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if (i >= pivotRange[0] && i <= pivotRange[1]) {
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return arr[i];
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} else if (i < pivotRange[0]) {
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return bfprt(arr, begin, pivotRange[0] - 1, i);
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} else {
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return bfprt(arr, pivotRange[1] + 1, end, i);
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}
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}
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/**
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* wikipedia: https://en.wikipedia.org/wiki/Median_of_medians .
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* @param arr an array.
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* @param begin begin num.
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* @param end end num.
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* @return median of medians.
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*/
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public static int medianOfMedians(int[] arr, int begin, int end) {
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int num = end - begin + 1;
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int offset = num % 5 == 0 ? 0 : 1;
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int[] mArr = new int[num / 5 + offset];
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for (int i = 0;i < mArr.length;i++) {
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mArr[i] = getMedian(arr, begin + i * 5, Math.min(end, begin + i * 5 + 4));
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}
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return bfprt(mArr, 0, mArr.length - 1, mArr.length / 2);
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}
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public static void swap(int[]arr, int i, int j) {
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int swap = arr[i];
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arr[i] = arr[j];
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arr[j] = swap;
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}
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public static int[] partition(int[] arr,int begin,int end,int num)
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{
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int small=begin-1;
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int cur=begin;
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int big=end+1;
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while(cur!=big)
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{
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if (arr[cur]<num)
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{
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swap(arr,++small,cur++);
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}else if (arr[cur]>num)
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{
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swap(arr,--big,cur);
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} else {
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cur++;
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}
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}
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int[] pivotRange=new int[2];
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pivotRange[0]=small+1;
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pivotRange[1]=big-1;
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return pivotRange;
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}
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public static int getMedian(int[] arr, int begin, int end) {
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insertionSort(arr, begin, end);
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int sum = begin + end;
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int mid = sum / 2 + (sum % 2);
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return arr[mid];
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}
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public static void insertionSort(int[] arr, int begin, int end) {
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if (arr == null || arr.length < 2) {
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return;
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}
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for (int i = begin + 1;i != end + 1;i++) {
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for (int j = i;j != begin;j--) {
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if (arr[j - 1] > arr[j]) {
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swap(arr, j - 1, j);
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} else {
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break;
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}
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}
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}
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}
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public static void main(String[] args) {
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int[] arr = { 11, 9, 1, 3, 9, 2, 2, 5, 6, 5, 3, 5, 9, 7, 2, 5, 5, 1, 9 };
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int[] minK = getMinKNumsByBFPRT(arr,5);
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System.out.println(Arrays.toString(minK));
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}
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}
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