全排列递归思路讲解

package cn.edu.ujn.nk;
 
public class FullPermutation {
/**
     * recursive method, used for the tree traversal.
     *  
     * @param inStr
     *            the elements need to be choose
     * @param pos
     *            the position of the elements we choose
     * @param parentData
     *            the elements have been chosen
     */  
    public void permutation(String inStr, int pos, StringBuffer parentData) {  
        if (inStr.length() == 0) {
            return;
        }
        if (inStr.length() == 1) {
            System.out.println("{" + inStr + "}");
            return;
        }
        // here we need a new buffer to avoid to pollute the other nodes.
        StringBuffer buffer = new StringBuffer();
        // copy from the parent node
        buffer.append(parentData.toString());
  
        // choose the element
        buffer.append(inStr.charAt(pos));
  
        // get the remnant elements.  
        String subStr = kickChar(inStr, pos);  
  
        // got one of the result  
        if (subStr.length() == 1) {  
            buffer.append(subStr);  
            System.out.println(buffer.toString());  
            return;  
        }  
  
        // here we use loop to choose other children.  
        for (int i = 0; i < subStr.length(); i++) {  
            permutation(subStr, i, buffer);  
        }  
  
    }  
  
    // a simple method to delete the element we choose  
    private String kickChar(String src, int pos) {  
        StringBuffer srcBuf = new StringBuffer();  
        srcBuf.append(src);  
        srcBuf.deleteCharAt(pos);  
        return srcBuf.toString();  
    }  
    
    public static void main(String args[]) {  
        FullPermutation p = new FullPermutation();
        StringBuffer buffer = new StringBuffer();
        String input = "ABCD";
        for (int i = 0; i < input.length(); i++) {  
            p.permutation(input, i, buffer);  
        }  
    }  
}