Sort Letters by Case

Given a string which contains only letters. Sort it by lower case first and upper case second.

Example

For "abAcD", a reasonable answer is "acbAD"

与将负数都放在前面，正数都放在后面的题目一样。

时间复杂度为O（n)

找到第一大写字母，记录其下标为i，则小写字母必定在i之后出现，在i之后找到第一个出现的小写字母j。交换i, j. 注意：i 到 j 之间的字母必定为大写。 依次进行即可。

 public class Solution {
     /**
      *@param chars: The letter array you should sort by Case
      *@return: void
      */
     public void sortLetters(char[] chars) {
         if (chars == null || chars.length == 0) {
             return;
         }
         int i = 0;
         for (i = 0; i < chars.length; i++) {
             if (chars[i] >= 'A' && chars[i] <= 'Z') {
                 break;
             }
         }
         for (int j = i + 1; j < chars.length; j++) {
             if (chars[j] >= 'a' && chars[j] <= 'z') {
                 char temp = chars[i];
                 chars[i] = chars[j];
                 chars[j] = temp;
                 i++;
             }
         }
         return;
     }
 }