504. Base 7

考察对N进制的理解，没啥好说的。
Given an integer, return its base 7 string representation.

Example 1:
Input: 100
Output: "202"
Example 2:
Input: -7
Output: "-10"

class Solution {
    public String convertToBase7(int num) {
        StringBuilder sb = new StringBuilder();
        int index = 0;
        if(num==0)
            return "0";
        if(num<0)
        {
            sb.append('-');
            index = num<0? 1:0;
            num=-num;
        }
        while(num!=0)
        {
            sb.insert(index,num%7);
            num=num/7;
        }
        return sb.toString();
        
    }
}