char[] c = {'a','b','c','d','e','f'};

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


public class Test {
    public static void main(String[] args) {
        char[] c = {'a','b','c','d','e','f'};
        
        //list保存找到的字符串
        List<String> list = new ArrayList<String>();
        //查找满足条件的字符串，并存入list
        for(int i=0; i<c.length; i++) {
            for(int j=0; j<c.length; j++) {
                if(i == j)
                    continue;
                for(int k=0; k<c.length; k++) {
                    if(i == k || j == k)
                        continue;
                    for(int l=0; l<c.length; l++) {
                        if(i == l || j == l || k == l)
                            continue;
                        for(int m=0; m<c.length; m++) {
                            if(i == m || j == m || k == m || l == m)
                                continue;
                            for(int n=0; n<c.length; n++) {
                                if(i == n || j == n || k == n || l == n || m == n)
                                    continue;
                                StringBuffer sb = new StringBuffer();
                                sb.append(c[i]);
                                sb.append(c[j]);
                                sb.append(c[k]);
                                sb.append(c[l]);
                                sb.append(c[m]);
                                sb.append(c[n]);
                                list.add(sb.toString());
                            }
                        }
                    }
                }
            }
        }
        
        //打印字符串的个数，换行打印字符串，每行10个
        System.out.println(list.size());
        int count = 0;
        for(int i=0; i<list.size(); i++) {
            System.out.print(list.get(i));
            count ++;
            if(count % 10 != 0)//如果没够10个，行尾加","号
                System.out.print(",");
            else//10个换行
                System.out.println();
        }
    }
}

------解决方案--------------------
Java code
public static void main(String[] args) {
    char[] origin = { 'a', 'b', 'c', 'd', 'e', 'f' };
    int length = origin.length;
    int[] tmp = new int[length];
    char[] res = new char[length];
    byte[] check = new byte[(length + 7) / 8];
    int index = 0, level = 0;
    while (true) {
        if (index >= 0 && index < length && level >= 0 && level < length) {
            if ((check[index / 8] & (1 << index % 8)) == 0) {
                tmp[level] = index;
                res[level] = origin[index];
                check[index / 8] |= (1 << index % 8);
                level++;
                index = 0;
                continue;
            } else {
                index++;
                continue;
            }
        }
        if (level >= length) {
            System.out.println(new String(res));
            level = length - 1;
            index = tmp[level];
            check[index / 8] &= ~(1 << index % 8);
            index++;
            continue;
        }
        if (index >= length) {
            level--;
            if (level < 0)
                break;
            index = tmp[level];
            check[index / 8] &= ~(1 << index % 8);
            index++;
            continue;
        }
    }
}

------解决方案--------------------
Java code
public static void main(String[] args) {
    char[] origin = { 'a', 'b', 'c', 'd', 'e', 'f' };
    int length = origin.length;
    int[] tmp = new int[length];
    char[] res = new char[length];
    boolean[] check = new boolean[length];
    int index = 0, level = 0;
    while (true) {
        if (index >= 0 && index < length && level >= 0 && level < length) {
            if (!check[index]) {
                tmp[level] = index;
                res[level] = origin[index];
                check[index] = true;
                level++;
                index = 0;
            } else {
                index++;
            }
        } else if (level >= length) {
            System.out.println(new String(res));
            level = length - 1;
            index = tmp[level];
            check[index] = false;
            index++;
        } else if (index >= length) {
            level--;
            if (level < 0)
                break;
            index = tmp[level];
            check[index] = false;
            index++;
        } else {
            break;
        }
    }
}