add solution of problem 6: zig zag conversion

Author: ppulse

ZigZagConversion6/README.md

@@ -0,0 +1,16 @@
+The string `"PAYPALISHIRING"` is written in a zigzag pattern on a given number of rows like this: (you may want to display this pattern in a fixed font for better legibility)
+```
+P   A   H   N
+
+A P L S I I G
+
+Y   I   R
+```
+And then read line by line: `"PAHNAPLSIIGYIR"`
+Write the code that will take a string and make this conversion given a number of rows:
+
+```
+string convert(string text, int nRows);
+```
+
+`convert("PAYPALISHIRING", 3)` should return `"PAHNAPLSIIGYIR"`.

ZigZagConversion6/Solution.java

@@ -0,0 +1,34 @@
+public class Solution {
+    public String convert(String s, int numRows) {
+        if (s == null || numRows == 1) {
+            return s;
+        }
+        
+        StringBuilder sb = new StringBuilder();
+        
+        
+        
+        for (int row = 0; row < numRows && row < s.length(); row++) {
+            int index = row;
+            int step1 = (numRows - 1 - row) * 2;
+            int step2 = row * 2;
+            sb.append(s.charAt(index));
+            
+            while (index < s.length()) {
+                if (step1 > 0) {
+                    index += step1;
+                    if (index < s.length())
+                        sb.append(s.charAt(index));
+                }
+                
+                if (step2 > 0) {
+                    index += step2;
+                    if (index < s.length())
+                        sb.append(s.charAt(index));
+                }
+            }
+        }
+        
+        return sb.toString();
+    }
+}