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| 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|