题目：输入数字n，按顺序打印出从1到最大的n位十进制数。比如输入3，则打印出1、2、3一直到最大的3位数999。

练习地址

https://leetcode-cn.com/problems/da-yin-cong-1dao-zui-da-de-nwei-shu-lcof/

方法1：遍历

public void print1ToMaxOfNDigits(int n) {
    if (n <= 0) {
        return;
    }
    char[] number = new char[n];
    for (int i = 0; i < n; i++) {
        number[i] = '0';
    }
    while (!increment(number)) {
        printNumber(number);
    }
}

private boolean increment(char[] number) {
    int takeOver = 0;
    int length = number.length;
    for (int i = length - 1; i >= 0; i--) {
        int sum = number[i] - '0' + takeOver;
        if (i == length - 1) {
            sum++;
        }
        if (sum >= 10) {
            if (i == 0) {
                return true;
            } else {
                sum -= 10;
                takeOver = 1;
                number[i] = (char) ('0' + sum);
            }
        } else {
            number[i] = (char) ('0' + sum);
            break;
        }
    }
    return false;
}

private void printNumber(char[] number) {
    boolean isBegin0 = true;
    int length = number.length;
    for (int i = 0; i < length; i++) {
        if (isBegin0 && number[i] != '0') {
            isBegin0 = false;
        }
        if (!isBegin0) {
            System.out.print(number[i]);
        }
    }
    if (!isBegin0) {
        System.out.println();
    }
}

方法2：递归

public void print1ToMaxOfNDigits(int n) {
    if (n <= 0) {
        return;
    }
    char[] number = new char[n];
    for (int i = 0; i < 10; i++) {
        number[0] = (char) (i + '0');
        print1ToMaxOfNDigitsRecursively(number, n, 0);
    }
}

private void print1ToMaxOfNDigitsRecursively(char[] number, int length, int index) {
    if (index == length - 1) {
        printNumber(number);
        return;
    }
    for (int i = 0; i < 10; i++) {
        number[index + 1] = (char) (i + '0');
        print1ToMaxOfNDigitsRecursively(number, length, index + 1);
    }
}

private void printNumber(char[] number) {
    boolean isBegin0 = true;
    int length = number.length;
    for (int i = 0; i < length; i++) {
        if (isBegin0 && number[i] != '0') {
            isBegin0 = false;
        }
        if (!isBegin0) {
            System.out.print(number[i]);
        }
    }
    if (!isBegin0) {
        System.out.println();
    }
}

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