feat: add 006

Author: Blankj

README.md

@@ -70,6 +70,7 @@
 | 2    | [Add Two Numbers][002]                   | Linked List, Math                |
 | 3    | [Longest Substring Without Repeating Characters][003] | Hash Table, Two Pointers, String |
 | 5    | [Longest Palindromic Substring][005]     | String                           |
+| 6    | [ZigZag Conversion][006]                 | String                           |
 | 8    | [String to Integer (atoi)][008]          | Math, String                     |
 | 15   | [3Sum][015]                              | Array, Two Pointers              |
 | 17   | [Letter Combinations of a Phone Number][017] | String, Backtracking             |
@@ -138,6 +139,7 @@
 [002]: https://github.com/Blankj/awesome-java-leetcode/blob/master/note/002/README.md
 [003]: https://github.com/Blankj/awesome-java-leetcode/blob/master/note/003/README.md
 [005]: https://github.com/Blankj/awesome-java-leetcode/blob/master/note/005/README.md
+[005]: https://github.com/Blankj/awesome-java-leetcode/blob/master/note/006/README.md
 [008]: https://github.com/Blankj/awesome-java-leetcode/blob/master/note/008/README.md
 [015]: https://github.com/Blankj/awesome-java-leetcode/blob/master/note/015/README.md
 [017]: https://github.com/Blankj/awesome-java-leetcode/blob/master/note/017/README.md

note/006/README.md

@@ -0,0 +1,112 @@
+# [Longest Palindromic Substring][title]
+
+## Description
+
+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"`.
+
+**Tags:** String
+
+
+## 思路 0
+
+题意是让你把字符串按波浪形排好，然后返回横向读取的字符串。
+
+听不懂的话，看下图应该就明白了：
+
+```
+                                                       
+0                           2n-2                        4n-4
+1                    2n-3   2n-1                 4n-5   4n-5
+2              2n-4         2n               4n-6       .
+.           .               .             .             .
+.       n+1                 .          3n-1             .
+n-2   n                     3n-4   3n-2                 5n-6
+n-1                         3n-3                        5n-5
+                                                       
+```
+
+那么我们可以根据上图找规律，可以看到最波峰和波谷是单顶点的，它们周期是 `2 * (n - 1)`，单独处理即可；中间的部分每个周期会出现两次，规律很好找，留给读者自己想象，不懂的可以结合以下代码。
+
+```java
+class Solution {
+    public String convert(String s, int numRows) {
+        if (numRows <= 1) return s;
+        int len = s.length();
+        char[] chars = s.toCharArray();
+        int cycle = 2 * (numRows - 1);
+        StringBuilder sb = new StringBuilder();
+        for (int j = 0; j < len; j += cycle) {
+            sb.append(chars[j]);
+        }
+        for (int i = 1; i < numRows - 1; i++) {
+            int step = 2 * i;
+            for (int j = i; j < len; j += step) {
+                sb.append(chars[j]);
+                step = cycle - step;
+            }
+        }
+        for (int j = numRows - 1; j < len; j += cycle) {
+            sb.append(chars[j]);
+        }
+        return sb.toString();
+    }
+}
+```
+
+
+## 思路 1
+
+另外一种思路就是开辟相应行数的 `StringBuilder` 对象，然后模拟波浪生成的样子分别插入到相应的 `StringBuilder` 对象，具体代码如下，比较直白简单。
+
+```java
+class Solution {
+    public String convert(String s, int numRows) {
+        if (numRows <= 1) return s;
+        int len = s.length();
+        char[] chars = s.toCharArray();
+        StringBuilder[] sbs = new StringBuilder[numRows];
+        for (int i = 0; i < numRows; i++) {
+            sbs[i] = new StringBuilder();
+        }
+        int i = 0;
+        while (i < len) {
+            for (int j = 0; j < numRows && i < len; ++j) {
+                sbs[j].append(chars[i++]);
+            }
+            for (int j = numRows - 2; j >= 1 && i < len; --j) {
+                sbs[j].append(chars[i++]);
+            }
+        }
+        for (i = 1; i < numRows; i++) {
+            sbs[0].append(sbs[i]);
+        }
+        return sbs[0].toString();
+    }
+}
+```
+
+
+## 结语
+
+如果你同我一样热爱数据结构、算法、LeetCode，可以关注我 GitHub 上的 LeetCode 题解：[awesome-java-leetcode][ajl]
+
+
+
+[title]: https://leetcode.com/problems/longest-palindromic-substring
+[ajl]: https://github.com/Blankj/awesome-java-leetcode

src/com/blankj/medium/_006/Solution.java

@@ -0,0 +1,63 @@
+package com.blankj.medium._006;
+
+/**
+ * <pre>
+ *     author: Blankj
+ *     blog  : http://blankj.com
+ *     time  : 2017/12/11
+ *     desc  :
+ * </pre>
+ */
+class Solution {
+
+//    public String convert(String s, int numRows) {
+//        if (numRows <= 1) return s;
+//        int len = s.length();
+//        char[] chars = s.toCharArray();
+//        int cycle = 2 * (numRows - 1);
+//        StringBuilder sb = new StringBuilder();
+//        for (int j = 0; j < len; j += cycle) {
+//            sb.append(chars[j]);
+//        }
+//        for (int i = 1; i < numRows - 1; i++) {
+//            int step = 2 * i;
+//            for (int j = i; j < len; j += step) {
+//                sb.append(chars[j]);
+//                step = cycle - step;
+//            }
+//        }
+//        for (int j = numRows - 1; j < len; j += cycle) {
+//            sb.append(chars[j]);
+//        }
+//        return sb.toString();
+//    }
+
+    public String convert(String s, int numRows) {
+        if (numRows <= 1) return s;
+        int len = s.length();
+        char[] chars = s.toCharArray();
+        StringBuilder[] sbs = new StringBuilder[numRows];
+        for (int i = 0; i < numRows; i++) {
+            sbs[i] = new StringBuilder();
+        }
+        int i = 0;
+        while (i < len) {
+            for (int j = 0; j < numRows && i < len; ++j) {
+                sbs[j].append(chars[i++]);
+            }
+            for (int j = numRows - 2; j >= 1 && i < len; --j) {
+                sbs[j].append(chars[i++]);
+            }
+        }
+        for (i = 1; i < numRows; i++) {
+            sbs[0].append(sbs[i]);
+        }
+        return sbs[0].toString();
+    }
+
+    public static void main(String[] args) {
+        Solution solution = new Solution();
+        System.out.println(solution.convert("PAYPALISHIRING", 3));// PAHNAPLSIIGYIR
+        System.out.println(solution.convert("ABCD", 4));
+    }
+}