30 最小n个数

 

public class test30{
    public static void main(String args[]){
        int array[]={3,2,5,1,4};
        int temp[] = findTopN(array,4);
        for(int i=0;i<temp.length;i++){
            System.out.print(temp[i]+",");
        }
    }
    public static int[] findTopN(int[] array, int n){
        int[] temp = new int[n];
        if(n<=0) return temp;
        int i = 0;
        int j = array.length-1;
        int index = partition(array,i,j);
        while(index != n){
            if(index>n){
                j = index-1;
                index = partition(array,i,j);
            }else{
                i = index+1;
                index = partition(array,i,j);
            }
        }
        
        for(int k=0;k<n;k++){
            temp[k]=array[k];
        }
        return temp;
        
    }
    private static int partition(int[] array, int i, int j) {
        int key = array[i];//i
        while(i<j){
            while(i<j&&array[j]>=key){//先j
                j--;
            }
            if(i<j){
                array[i++] = array[j];
            }
            while(i<j&&array[i]<=key){//后i
                i++;
            }
            if(i<j){
                array[j--] = array[i]; 
            }
            
        }
        array[i] = key;
        return i;
    }
}

 海量数据时：