int[] arr;  //注意int[5] arr；错误int arr[];int[] arr = {1, 2, 3, 4, 5}; int[] arr = new int[5];

int[] arr[]; //此种方式最好理解int arr[][];int[][] arr;int arr[][] = {{1,2},{3,4},{5,6,7}};int arr[][] = new int[3][3]

public class BitCount {//方式一：移位加计数器  注意java中没有无符号整形public static int bitCount(int n){ int count = 0; while(n > 0){ if((n & 1) == 1){ count++; } n >>= 1; } return count; }

public static int bitCount1(int n){int count = 0; unsigned int flag = 1; while(flag){ if(n&flag){ count++; } flag = flag << 1; } return count; }

    //方式三：使用n = n & (n-1)public static int bitCount2(int n){ int count = 0; while(n > 0){ n &= n-1; //消除最右边的1,和第一种方式等价，只是这个的执行次数由数字中的1决定 count++; } return count; }

    public static void main(String[] args) {System.out.println("3: "+ bitCount(3));System.out.println("8: "+ bitCount(8)); System.out.println("11: "+ bitCount(11)); System.out.println("6: "+ bitCount2(6)); System.out.println("7: "+ bitCount2(7)); System.out.println("13: "+ bitCount2(13)); } }

3: 2
8: 1
11: 3
6: 2
7: 3
13: 3

public static int partition(int[] data, int start, int end){ if(data == null || start < 0 || end >= data.length){ throw new InvalidParameterException(); } Random rand = new Random(); int index = rand.nextInt(end-start+1)+start; swap(data, index, end); int borderPos = start-1; //记录小于部分和大于部分的分界点,指向最后一个小于的值 for(index = start; index < end; ++index){ if(data[index] < data[end]){ borderPos++; if(borderPos != index){ swap(data, borderPos, index); } } } borderPos++; swap(data, borderPos, end); return borderPos; } public static void swap(int[] data, int pos1, int pos2){ int tmp = data[pos1]; data[pos1] = data[pos2]; data[pos2] = tmp; }

    public static int[] getSmallestNum(int[] numbers, int k){ if(numbers == null || k <= 0 || k > numbers.length){ throw new InvalidParameterException(); } int start = 0, end = numbers.length -1, index = partition(numbers, start, end); while(index != k-1){ if(index > k-1){ end = index - 1; index = partition(numbers, start, end); }else{ start = index + 1; index = partition(numbers, start, end); } } int[] result = new int[k]; for(int i = 0; i < k; i++){ result[i] = numbers[i]; } return result; } 

public static void main(String[] args) {int[] arr = new int[20]; for(int i=0; i<20; i++){ arr[i] = (int) (Math.random()*100); } for(int i=0,len=arr.length; i<len; i++){ System.out.print(arr[i]+" "); } int[] result = getSmallestNum(arr,5); System.out.println(""); for(int i=0,len=result.length; i<len; i++){ System.out.print(result[i]+" "); }

91 69 75 29 69 55 80 44 63 19 36 53 62 45 97 52 8 93 34 38
29 19 8 34 36 

    public static int[] getSmallestNums1(int[] numbers, int k){ //参数检查 if(numbers == null || k <= 0){ throw new InvalidParameterException(); } //存储大根堆，初始值为numbers的前k个数 int[] heap = new int[k]; for(int i = 0; i < k; i++){ heap[i] = numbers[i]; } int rootIndex = k/2 -1; while(rootIndex >= 0){ reheap(heap, rootIndex, k-1); rootIndex--; } for(int i = k, len = numbers.length; i < len; i++){ //如果数值比最大值小，替换最大值 if(numbers[i] < heap[0]){ heap[0] = numbers[i]; reheap(heap, 0, k-1); } } return heap; } //重建大根堆 public static void reheap(int[] heap, int rootIndex, int end){ int node = heap[rootIndex]; int leftIndex = rootIndex*2 +1; boolean done = false; while(!done && leftIndex <= end){ int index = leftIndex; if(leftIndex+1 <= end){ int rightIndex = leftIndex + 1; if(heap[rightIndex] > heap[leftIndex]){ index = rightIndex; } } if(node < heap[index]){ heap[rootIndex] = heap[index]; rootIndex = index; leftIndex = rootIndex*2 + 1; }else{ done = true; } } heap[rootIndex] = node; }

public static void main(String[] args) {int[] arr = new int[20]; for(int i=0; i<20; i++){ arr[i] = (int) (Math.random()*100); } for(int i=0,len=arr.length; i<len; i++){ System.out.print(arr[i]+" "); } int[] result = getSmallestNums1(arr,5); System.out.println(""); for(int i=0,len=result.length; i<len; i++){ System.out.print(result[i]+" "); } }

91 13 44 34 3 57 62 89 57 62 45 1 40 71 76 18 1 28 32 55
18 13 1 1 3