面试题 01.03. URL化

解题一: 先计算长度，然后再遍历
class Solution {
    public String replaceSpaces(String S, int length) {
        int ln = 0;                                       
        for (int i = 0; i < length; i++) {                
            if (S.charAt(i) == ' ') {                     
                ln += 3;                                  
            } else {                                      
                ln ++;                                    
            }                                             
        }                                                 
        char[] c = new char[ln];                          
        int j = 0;                                        
        for (int i = 0; i < length; i++) {                
            if (S.charAt(i) == ' ') {                     
                c[j++] = '%';                             
                c[j++] = '2';                             
                c[j++] = '0';                             
            } else {                                      
                c[j++] = S.charAt(i);                     
            }                                             
        }                                                 
        return new String(c);                             
    }
}

//runtime:16 ms
//memory:45.8 MB

解题二
class Solution {
    public String replaceSpaces(String S, int length) {
        return S.substring(0, length).replace(" ", "%20");
    }
}

//runtime:11 ms
//memory:47.1 MB
解题三
class Solution {
    public String replaceSpaces(String S, int length) {
        StringBuilder  sb = new StringBuilder();
        for (int i = 0; i < length; i++) {
            if (S.charAt(i) == ' ') {
                sb.append("%20");
            } else {
                sb.append(S.charAt(i));
            }
        }
        return sb.toString();
    }
}

//runtime:19 ms
//memory:46.1 MB