模擬一個trim方法，去除字符串兩端的空格。

@Test
public void test1() {
    String str1 = "   h e llo wor ld    ";
    int begin = 0;
    int end = str1.length() - 1;
    while (begin <= end && str1.charAt(begin) == ' '){
        begin ++;
    }
    while (begin <= end && str1.charAt(end) == ' '){
        end --;
    }
    System.out.println(begin);
    System.out.println(end);
    String substring = str1.substring(begin, end+1);
    System.out.println(substring);
}