记录贴，互勉

154. Find Minimum in Rotated Sorted Array II
For this question same as previous, but we need to take care the number that repeated, which we need to search both side of the array, and also prevent from overflow, we need to use l + ( r -l ) /2 instead of  (r + l) /2

912. Sort an Array (Merge sort)
time complexity: O(n*(logn))     每层排序n个数take O(n). 有log(n)层

315. Count of Smaller Numbers After Self  //Hard
MergeSort solution
1. Create a class to store the value and its index on the array
2. Use an array to store the mapping of the index and counter for smaller values on the right
3. Use the merge sort method to count the value as smaller
4. Because the left array is sorted in ascending order, we can count how many smaller numbers prevent repeating checks.

Divide and conquer（分治）

List（链表）
2. Add Two Numbers (Medium

445. Add Two Numbers II
ReverLinkList.

24. Swap Nodes in Pairs  Medium

206. Reverse Linked List

141. Linked List Cycle
two pointer

142. Linked List Cycle II
Find a cycle and the fast and slow meet point