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].

　　

解题：题意很容易理解，将十进制整型数转化为七进制的数，并以字符串的形式返回。先看我写的第一种方法，比较繁琐，也利用了StringBuffer和堆栈，代码如下：

 class Solution {
     public String convertToBase7(int num) {
         Stack<Integer>stack = new Stack<Integer>();
         boolean isNagative = false;
         if(num < 0){
             isNagative = true;
             num = -num;
         }
         while(num != 0){
             stack.push(num % 7);
             num /= 7;
         }
         StringBuffer result = new StringBuffer("");

         while(!stack.isEmpty()) result.append(String.valueOf(stack.pop()));
         if(result.length() == 0)
             return String.valueOf(0);
         else if(isNagative)
             return '-'+result.toString();
         else return result.toString();
     }
 }

稍微改进一下，去掉栈和StringBuffer，直接使用String及其性质，速度会快很多，代码如下：

 class Solution {
     public String convertToBase7(int num) {
         boolean isNagative = false;
         String result = "";
         if(num < 0){
             isNagative = true;
             num = -num;
         }
         while(num != 0){
             result = String.valueOf(num % 7) + result;
             num /= 7;
         }
         if(result.length() == 0)
             return String.valueOf(0);
         else if(isNagative)
             return '-'+result;
         else return result;
     }
 }

当然也可以这样，虽然有点投机取巧：

  public String convertToBase7(int num) {
         return Integer.toString(num, 7);
  }