字符串--替换空格

题意： 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."

代码：

public class ReplaceSpace {
    public static String replace(String s) {
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) == ' ') {
                sb.append("  ");
            }
        }

        int oldSize = s.length();
        s = s + sb.toString();
        int newSize = s.length();
        char[] chars = s.toCharArray();

        for (int i = oldSize -1, j = newSize - 1;  i < j; i--, j--) {
            if (chars[i] == ' ') {
                chars[j--] = '0';
                chars[j--] = '2';
                chars[j] = '%';
            } else {
                chars[j] = chars[i];
            }
        }
        return new String(chars);
    }


    public static String replaceSpace(String s) {
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) == ' ') {
                sb.append("%20");
            } else {
                sb.append(s.charAt(i));
            }
        }
        return sb.toString();
    }

    public static void main(String[] args) {
        String s = "We are happy.";
        System.out.println(replaceSpace(s));
        System.out.println(replace(s));
    }
}