剑指 Offer 05. 替换空格

请实现一个函数，把字符串 s 中的每个空格替换成”%20”。

示例 1：

输入：s = “We are happy.”

输出：”We%20are%20happy.”

限制：

0 <= s 的长度 <= 10000

1. 系统API

   class Solution {
    public String replaceSpace1(String s) {
       if (s == null || s == "") {
          return "";
    }
    return s.replaceAll(" ", "%20");
    }
   }

2. Char 数组

   class Solution {
   public String replaceSpace(String s) {
            if (s == null || s == "") {
                return "";
            }
            StringBuilder result = new StringBuilder();
            char [] arr = s.toCharArray();
            for (int i = 0; i < arr.length; i++) {
                if (arr[i] == ' ') {
                    result.append("%20");
                } else {
                result.append(arr[i]);
                }
            }
            return result.toString();
        }
   }

3. char 数组

    class Solution {
        public String replaceSpace(String s) {
            if (s == null || s == "") {
                return "";
            }
            int temp = 0;
            int len = s.length();
            char[] arr = new char[len * 3];
            for (int i = 0; i < len; i++) {
                if (s.charAt(i) == ' ') {
                    arr[temp++] = '%';
                    arr[temp++] = '2';
                    arr[temp++] = '0';
                } else {
                    arr[temp++] = s.charAt(i);
                }
            }
            return new String(arr, 0, temp);
        }
    }