Print Numbers by Recursion

题目描述

Print numbers from 1 to the largest number with N digits by recursion.
Example
Given N = 1, return [1,2,3,4,5,6,7,8,9].
Given N = 2, return [1,2,3,4,5,6,7,8,9,10,11,12,…,99].

题目解答

解题思路

代码实现

//这里没有考虑超大数的情况
public class Solution {
    /** * @param n: An integer. */
    public List<Integer> numbersByRecursion(int n) {
        List<Integer> result = new ArrayList<Integer>();
        if (n <= 0) {
            return result;
        }
        helper(n, result);
        return result;
    }

    private void helper(int n, List<Integer> ret) {
        if (n == 0) return;
        helper(n - 1, ret);
        // current base such as 10, 20, 30...
        int base = (int)Math.pow(10, n - 1);
        // get List size before for loop
        int size = ret.size();
        for (int i = 1; i < 10; i++) {
            // add 10, 100, 1000...
            ret.add(i * base);
            for (int j = 0; j < size; j++) {
                // add 11, 12, 13...
                ret.add(ret.get(j) + base * i);
            }
        }
    }
}