全排列/组合

package 笔试;

import java.util.ArrayList;
import java.util.List;

public class 字符串全排列 {
    
    public static void main(String[] args) {
        char[] str = new char[] {'A','B','C'};
        System.out.println(getString(str));
        System.out.println(getString(str).size());
    }

    public static List<String> getString(char[] str) {
        List<String> result = new ArrayList<>();
        if (str == null || str.length == 0) {
            return result;
        }
        StringBuilder sb = new StringBuilder();
        boolean[] flag = new boolean[str.length];
        doCombination(sb, result, str, flag);
        return result;
    }
    
    //全排列
    private static void doPermutation(StringBuilder sb, List<String> result, char[] str) {
        if (sb.length() == str.length) {
            result.add(sb.toString());
            return;
        }
        for (int i = 0; i < str.length; i++) {
            sb.append(str[i]);
            doPermutation(sb, result, str);
            sb.deleteCharAt(sb.length() - 1);
        }
    }
    
    //组合
    private static void doCombination(StringBuilder sb, List<String> result, char[] str, boolean[] flag) {
        if (sb.length() == str.length) {
            result.add(sb.toString());
            return;
        }
        for (int i = 0; i < str.length; i++) {
            if (flag[i]) {
                continue;
            }
            sb.append(str[i]);
            flag[i] = true;
            doCombination(sb, result, str, flag);
            sb.deleteCharAt(sb.length() - 1);
            flag[i] = false;
        }
    }
}