替换空格

s = "We are happy."

"We%20are%20happy."

public String replaceBlank1(String s) {
    if (s == null) {
        return null;
    }
    StringBuilder stringBuilder = new StringBuilder();
    char c = ' ';
    for (int i = 0; i < s.length(); i++) {
        c = s.charAt(i);
        if (c == ' ') {
            stringBuilder.append("%20");
        } else {
            stringBuilder.append(c);
        }
    }
    return stringBuilder.toString();
}

public String replaceBlank2(String s) {
    if (s == null) {
        return null;
    }
    StringBuilder stringBuilder = new StringBuilder(s);
    int numBlank = 0;
    int length = stringBuilder.length();
    for (int i = 0; i < length; i++) {
        if (stringBuilder.charAt(i) == ' ') {
            numBlank++;
        }
    }
    length = length - 1;
    int newStringLength = length + 2 * numBlank;
    stringBuilder.setLength(newStringLength + 1);
    char c = ' ';
    while (length < newStringLength) {
        c = s.charAt(length);
        if (c != ' ') {
            stringBuilder.setCharAt(newStringLength--, c);
        } else {
            stringBuilder.setCharAt(newStringLength--, '0');
            stringBuilder.setCharAt(newStringLength--, '2');
            stringBuilder.setCharAt(newStringLength--, '%');
        }
        length--;
    }
    return stringBuilder.toString();
}