504. Base 7

Given an integer, return its base 7 string representation.

Example 1:
Input: 100
Output: "202"

Example 2:
Input: -7
Output: "-10"

Note: The input will be in range of [-1e7, 1e7].

Solution：

思路：
Time Complexity: O(N) Space Complexity: O(N)

Solution Code：

class Solution {
    public String convertTo7(int num) {
    if (num < 0) {
        return '-' + convertTo7(-num);
    }
    if (num < 7) {
        return num + "";
    }
    return convertTo7(num / 7) + num % 7;
}
}