Cracking the coding interview--Q12.4

题目

原文：

You have an array with all the numbers from 1 to N, where N is at most 32,000. The array may have duplicate entries and you do not know what N is. With only 4KB of memory available, how would you print all duplicate elements in the array?

译文：

你有一个存有1到N所有数据的数组，N最大为32,000。这个数组可能有重复的元素，你并不知道N是什么。仅仅4kb的内存可用，怎样打印出这个数组中所有重复的元素。

解答

4kb的内存有4*1024*8=32768>32000,所以可以用Bit Map来处理，代码如下：

class Q12_4{

	public static void print_duplicates(int[] a,int n,int bitsize){
		BitMap bm=new BitMap(bitsize);
		for(int i=0;i<n;++i){
			if(bm.get(a[i]-1))  
				System.out.println(a[i]);
			else
				bm.set(a[i]-1);
		}
	}
	public static void main(String[] args){
		int a[] = {
        	1,2,3,4,5,32000,7,8,9,10,11,1,2,13,15,16,32000,11,5,8
    	};
    	print_duplicates(a,20,32000);
	}
}
class BitMap{
	public int[] bits;
	public BitMap(int size){
		bits=new int[size];
	}
	public boolean get(int pos){
		return (bits[pos/32]&(1<<(pos&0x1f)))!=0;
	}
	public void set(int pos){
		bits[pos/32]|=(1<<(pos&0x1f));
	}
}

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