cmn.java

用java实现了数学上的排列组合问题

import java.util.ArrayList;

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 * @author yjfeng
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public class Cmn {
	
	private String mString;
	private int n;
	
	
	public Cmn(String mString,int n){
		this.mString = mString;
		this.n = n;
	}
	
	private int getTheMax(){
		int lenth = mString.length();
		double maxvalue = 0;
		for(int i = 0;i < lenth; i ++ ){
			maxvalue = maxvalue + Math.pow(2, i);
		}
		return (int)maxvalue;
	}
	
	private int count1(String binaryStr){
		int count = 0;
		for(int i = 0;i < binaryStr.length();i ++){
			if(binaryStr.charAt(i) == '1'){
				count ++;
			}
		}
		return count;
	}

	
	private String[] getCmnIndexStr(){
		int maxvalue = getTheMax();		
		ArrayList list = new ArrayList();
		for(int j = 1;j <= maxvalue;j ++){
			String binaryStr = Integer.toBinaryString(j);
			if(count1(binaryStr) == n){
				list.add(appendZero(binaryStr));
			}			
		}
		return (String[])list.toArray(new String[0]);
	}
	
	private String appendZero(String str){
		int len = mString.length();
		if(str.length() < len){
			int sublen = len - str.length();
			String appendStr = "";
			for(int i = 0;i < sublen;i ++){
				appendStr = appendStr + "0";
			}
			str = appendStr + str;
			
		}
		return str;
	}

	
	private String getStrByIndexStr(String indexStr,String mstr){
		String result = "";
		for(int i = 0;i < indexStr.length();i ++){
			if(indexStr.charAt(i) == '1'){
				result = result + mstr.charAt(i);
			}
		}
		return result;
	}
	
	public String[] getCmnString(){
		if(n == 0){
			return new String[0];
		}else if(n == mString.length()){
			return new String[]{mString};
		}
		
		String[] indexStr = getCmnIndexStr();
		String[] cmnStr = new String[indexStr.length];
		for(int i = 0;i < indexStr.length;i ++){
			cmnStr[i] = getStrByIndexStr(indexStr[i],mString);
		}
		return cmnStr;
	}
	
	

	
	
	public static void main(String[] a){
		Cmn cmn = new Cmn("123456",3);
		String[] str = cmn.getCmnString();
		for(int i = 0;i < str.length;i ++){
			System.out.println(str[i]);
		}
	}

}