算法:替换空格

public class Solution {
    public String replaceSpace(StringBuffer str) {
        if (str == null) return null;
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < str.length(); ++i) {
            char c = str.charAt(i);
            if (c == ' ') {
                sb.append("%20");
            } else {
                sb.append(c);
            }
        }
        return sb.toString();
    }
}