思路：直接使用int進行遍歷，int有邊界（使用陣列模擬，或者使用全排列）

1 陣列模擬法

public class Solution {
    public void printToMaxOfNDedits(int n){
        if(n < 0)
            return;
        if (n ==0 ){
            System.out.println("0");
        }
        char [] num = new char[n];
        //初始化
        for (int i = 0; i < num.length; i++) {
            num[i] = '0';
        }
        while(!increment(num)){
            print(num);
        }
    }
    private static boolean increment(char [] num){
        boolean isEnd = false;
        int len = num.length;
        int nTakeOver = 0;//進位
        for (int i = len - 1; i >= 0 ; i--) {
            int nSum = num[i] - '0' + nTakeOver;//當前值
            //只有是個位時，數值加1
            if(i == len - 1)
                nSum++;
            //滿10進1
            if (nSum > 9){
                if(i == 0){
                    //超過n位
                    isEnd = true;
                }else {
                    nTakeOver = 1;
                    num[i] = '0';
                }
            }else {
                num[i] = (char) ('0' + nSum);
                break;
            }
        }
        return isEnd;
    }
    private static void print(char [] num){
        StringBuilder str = new StringBuilder();
        boolean isBegin = false;
        int len = num.length;
        for (int i = 0; i < len; i++) {
            if(num[i] != '0' && !isBegin){
                isBegin = true;
            }
            if (isBegin){
                str.append(String.valueOf(num[i]));
            }
        }

        if(str.toString().matches("^9+$")){
            System.out.println(str.toString()+" ");
        } else {
            System.out.print(str.toString()+" ");
        }
    }

}
 

2 全排列代替法

public class Solution {
    public void printToMaxOfNDedits(int n){
        if(n < 0)
            return;
        if (n == 0 ){
            System.out.println("0");
            return;
        }
        char [] num = new char[n];
        //初始化
        for (int i = 0; i < num.length; i++) {
            num[i] = '0';
        }
        for (int i = 0; i < 10; i++) {
            num[0] = (char) ('0' + i);
            printToMaxOfNDeditsRecurstively(num, n, 0);
        }
    }
    private static void printToMaxOfNDeditsRecurstively(char [] num,int length, int index){
        if(index == length - 1){
            print(num);
            return;
        }
        for (int i = 0; i < 10; i++) {
            num[index + 1] = (char)(i + '0');
            printToMaxOfNDeditsRecurstively(num, length, index + 1);
        }
    }
    private static void print(char [] num){
        StringBuilder str = new StringBuilder();
        boolean isBegin = false;
        int len = num.length;
        for (int i = 0; i < len; i++) {
            if(num[i] != '0' && !isBegin){
                isBegin = true;
            }
            if (isBegin){
                str.append(String.valueOf(num[i]));
            }
        }
        if(str.toString().matches("^9+$")){
            System.out.println(str.toString()+" ");
        } else {
            System.out.print(str.toString()+" ");
        }
    }
}
 

擴充套件：char 最多能標識256個字元，十進位制數：只用到10個，沒有充分利用記憶體，有更高效的方式嗎？有改char為byte

public class Solution {
    public void printToMaxOfNDedits(int n){
        if(n < 0)
            return;
        if (n ==0 ){
            System.out.println("0");
        }
        byte [] num = new byte[n];
        //初始化
        for (int i = 0; i < num.length; i++) {
            num[i] = 0;
        }
        while(!increment(num)){
            print(num);
        }
    }
    private static boolean increment(byte [] num){
        boolean isEnd = false;
        int len = num.length;
        byte nTakeOver = 0;//進位
        for (int i = len - 1; i >= 0 ; i--) {
            byte nSum = (byte) (num[i]  + nTakeOver);//byte相加，需要強轉
            //只有是個位時，數值加1
            if(i == len - 1)
                nSum++;
            //滿10進1
            if (nSum > 9){
                if(i == 0){
                    //超過n位
                    isEnd = true;
                }else {
                    nTakeOver = 1;
                    num[i] = 0;
                }
            }else {
                num[i] = nSum;
                break;
            }
        }
        return isEnd;
    }
    private static void print(byte [] num){
        StringBuilder str = new StringBuilder();
        boolean isBegin = false;
        int len = num.length;
        for (int i = 0; i < len; i++) {
            if(num[i] != 0 && !isBegin){
                isBegin = true;
            }
            if (isBegin){
                str.append(String.valueOf(num[i]));
            }
        }

        if(str.toString().matches("^9+$")){
            System.out.println(str.toString()+" ");
        } else {
            System.out.print(str.toString()+" ");
        }
    }
}