Add 007 and 009 Java code.

Author: qiyuangong

README.md

@@ -6,14 +6,14 @@
 
 | # | Title | Solution | Basic idea (One line) |
 |---| ----- | -------- | --------------------- |
-| 1 | [Two Sum](https://leetcode.com/problems/two-sum/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/001_Two_Sum.py) | 1. Hash O(n) and O(n) space<br>2. Sort and search with two points O(n) and O(1) space |
-| 2 | [Add Two Numbers](https://leetcode.com/problems/add-two-numbers/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/002_Add_Two_Numbers.py) | Note the carry from lower digit. |
-| 3 | [Longest Substring Without Repeating Characters](https://leetcode.com/problems/longest-substring-without-repeating-characters/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/003_Longest_Substring_Without_Repeating_Characters.py) |1. Check every possible substring O(n^2) <br>2. Remember the character index and current check pos, if character index >= current pos, then there is duplicate |
-| 4 | [Median of Two Sorted Arrays](https://leetcode.com/problems/median-of-two-sorted-arrays/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/004_Median_of_Two_Sorted_Arrays.py) | 1. Merge two sorted lists and compute median, O(m + n) and O(m + n)<br>2. An extension of median of two sorted arrays of equal size problem|
-| 5 | [Longest Palindromic Substring](https://leetcode.com/problems/longest-palindromic-substring/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/005_Longest_Palindromic_Substring.py) | [Background knowledge](http://en.wikipedia.org/wiki/Longest_palindromic_substring)<br>1. DP if s[i]==s[j] and P[i+1, j-1] then P[i,j]<br>2. A palindrome can be expanded from its center<br>3. Manacher algorithm|
-| 7 | [Reverse Integer](https://leetcode.com/problems/reverse-integer/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/007_Reverse_Integer.py) | Overflow when the result is greater than 2147483647
-| 8 | [String to Integer (atoi)](https://leetcode.com/problems/string-to-integer-atoi/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/008_String_to_Integer(atoi).py) | Overflow, Space, and negative number |
-| 9 | [Palindrome Number](https://leetcode.com/problems/palindrome-number/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/009_Palindrome_Number.py)| Get the len and check left and right with 10^len, 10 |
+| 1 | [Two Sum](https://leetcode.com/problems/two-sum/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/001_Two_Sum.py) [Java](https://github.com/qiyuangong/leetcode/blob/master/java/001_Two_Sum.java) | 1. Hash O(n) and O(n) space.<br>2. Sort and search with two points O(n) and O(1) space. |
+| 2 | [Add Two Numbers](https://leetcode.com/problems/add-two-numbers/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/002_Add_Two_Numbers.py) [Java](https://github.com/qiyuangong/leetcode/blob/master/java/002_Add_Two_Numbers.java) | Take care of the carry from lower digit. |
+| 3 | [Longest Substring Without Repeating Characters](https://leetcode.com/problems/longest-substring-without-repeating-characters/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/003_Longest_Substring_Without_Repeating_Characters.py) [Java](https://github.com/qiyuangong/leetcode/blob/master/java/003_Longest_Substring_Without_Repeating_Characters.java) |1. Check every possible substring O(n^2) <br>2. Remember the character index and current check pos, if character index >= current pos, then there is duplicate |
+| 4 | [Median of Two Sorted Arrays](https://leetcode.com/problems/median-of-two-sorted-arrays/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/004_Median_of_Two_Sorted_Arrays.py) [Java](https://github.com/qiyuangong/leetcode/blob/master/java/004_Median_of_Two_Sorted_Arrays.java) | 1. Merge two sorted lists and compute median, O(m + n) and O(m + n)<br>2. An extension of median of two sorted arrays of equal size problem|
+| 5 | [Longest Palindromic Substring](https://leetcode.com/problems/longest-palindromic-substring/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/005_Longest_Palindromic_Substring.py) [Java](https://github.com/qiyuangong/leetcode/blob/master/java/005_Longest_Palindromic_Substring.java) | [Background knowledge](http://en.wikipedia.org/wiki/Longest_palindromic_substring)<br>1. DP if s[i]==s[j] and P[i+1, j-1] then P[i,j]<br>2. A palindrome can be expanded from its center<br>3. Manacher algorithm|
+| 7 | [Reverse Integer](https://leetcode.com/problems/reverse-integer/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/007_Reverse_Integer.py) [Java](https://github.com/qiyuangong/leetcode/blob/master/java/007_Reverse_Integer.java) | Overflow when the result is greater than 2147483647 or less than -2147483648.
+| 8 | [String to Integer (atoi)](https://leetcode.com/problems/string-to-integer-atoi/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/008_String_to_Integer(atoi).py) [Java](https://github.com/qiyuangong/leetcode/blob/master/java/008_String_to_Integer(atoi).java) | Overflow, Space, and negative number |
+| 9 | [Palindrome Number](https://leetcode.com/problems/palindrome-number/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/009_Palindrome_Number.py) [Java](https://github.com/qiyuangong/leetcode/blob/master/java/009_Palindrome_Number.java) | Get the len and check left and right with 10^len, 10 |
 | 12 | [Integer to Roman](https://leetcode.com/problems/integer-to-roman/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/012_Integer_to_Roman.py) | [Background knowledge](http://www.rapidtables.com/convert/number/how-number-to-roman-numerals.htm) Just like 10-digit number, divide and minus |
 | 13 | [Roman to Integer](https://leetcode.com/problems/roman-to-integer/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/013_Roman_to_Integer.py) | Add all curr, if curr > prev, then need to subtract 2 * prev |
 | 15 | [3Sum](https://leetcode.com/problems/3sum/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/015_3Sum.py) | 1. Sort and O(n^2) search with three points <br>2. Multiple Two Sum (Problem 1) |

java/007_Reverse_Integer.java

@@ -0,0 +1,13 @@
+class Solution {
+    public int reverse(int x) {
+        if (x == 0) return 0;
+        long res = 0;
+        while (x != 0) {
+            res = res * 10 + x % 10;
+            if (res > Integer.MAX_VALUE || res < Integer.MIN_VALUE)
+                return 0;
+            x /= 10;
+        }
+        return (int) res;
+    }
+}

java/009_Palindrome_Number.java

@@ -0,0 +1,41 @@
+class Solution {
+    public boolean isPalindrome(int x) {
+        if (x < 0) 
+            return false;
+        int temp = x;
+        int len = 0;
+        while (temp != 0) {
+            temp /= 10;
+            len ++;
+        }
+        temp = x;
+        int left, right;
+        for (int i = 0; i < len / 2; i++) {
+            right = temp % 10;
+            left = temp / (int) Math.pow(10, len - 2 * i - 1);
+            left = left % 10;
+            if (left != right)
+                return  false;
+            temp /= 10;
+        }
+        return true;
+    }
+
+    // Leetcode book
+    public boolean isPalindrome(int x) {
+        if (x < 0) return false;
+        int div = 1;
+        while ( x / div >= 10) {
+            div *= 10;
+        }
+        while (x !=0) {
+            int l = x / div;
+            int r = x % 10;
+            if (l != r) return false;
+            // Remove left and right number
+            x = (x % div) / 10;
+            div /= 100;
+        }
+        return true;
+    }
+}