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：

 1 class Solution {
 2     public String convertToBase7(int num) {
 3         Stack<Integer>stack = new Stack<Integer>();
 4         boolean isNagative = false;
 5         if(num < 0){
 6             isNagative = true;
 7             num = -num;
 8         }
 9         while(num != 0){
10             stack.push(num % 7);
11             num /= 7;
12         }
13         StringBuffer result = new StringBuffer("");
14         
15         while(!stack.isEmpty()) result.append(String.valueOf(stack.pop()));
16         if(result.length() == 0) 
17             return String.valueOf(0);
18         else if(isNagative)
19             return '-'+result.toString();
20         else return result.toString();
21     }
22 }

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：

 1 class Solution {
 2     public String convertToBase7(int num) {
 3         boolean isNagative = false;
 4         String result = "";
 5         if(num < 0){
 6             isNagative = true;
 7             num = -num;
 8         }
 9         while(num != 0){
10             result = String.valueOf(num % 7) + result;
11             num /= 7;
12         } 
13         if(result.length() == 0) 
14             return String.valueOf(0);
15         else if(isNagative)
16             return '-'+result;
17         else return result;
18     }
19 }

当然也可以这样，虽然有点投机取巧：

1  public String convertToBase7(int num) {
2         return Integer.toString(num, 7);
3  }