ZigZag Conversion

 1 public class Solution {
 2     public String convert(String s, int numRows) {
 3         // illegal parameters
 4         if(s == null || s.length() == 0 || numRows <= 0) {
 5             return s;
 6         }
 7         
 8         if(s.length() <= numRows || numRows == 1) {
 9             return s;
10         }
11         
12         
13         // get column numbers of array
14         int numCols = 0;
15         int len = s.length();
16         while(len > 0) {
17             len -= numRows * 2 - 2;
18             numCols += numRows - 1;
19         }
20         
21         StringBuilder res = new StringBuilder();
22         Character[][] array = new Character[numRows][numCols];
23         boolean isDown = true;
24         int rowIndex = 0;
25         int colIndex = 0;
26         
27         for(int i = 0; i < s.length(); i++) {
28             Character c = s.charAt(i);
29             array[rowIndex][colIndex] = c;
30             if(isDown) {
31                 // going down direction
32                 rowIndex++;
33                 if(rowIndex == numRows) {
34                     // time to change to lean direction
35                     isDown = false;
36                     rowIndex -= 2;
37                     colIndex++;
38                 }
39             } else {
40                 // going lean direction
41                 if(rowIndex == 0) {
42                     // time to change to down direction
43                     isDown = true;
44                     rowIndex++; 
45                 } else {
46                     rowIndex--;
47                     colIndex++;
48                 }
49             }
50         }
51         
52         for(int i = 0; i < numRows; i++) {
53             for(int j = 0; j < numCols; j++) {
54                 Character c = array[i][j];
55                 if(c == null) {
56                     continue;
57                 } else {
58                     res.append(c);
59                 }
60             }
61         }
62         
63         return res.toString();
64     }
65 }