1.Two Sum

• array null, length and bound check
• use hashmap to decrease time complexity
• two pointer to solve sorted array problems
• consider no result condition

4.Median of Two Sorted Arrays

• abstract the problem to getKthMin problem
• consider even and odd length of array
• consider k VS length in getKthMin function
• compare k/2 - 1 position in both array to abort half of k elements in either array
• log(m+n) time complexity

11. Container With Most Water

• two pointer to solve unsorted problems
• two pointer to find max problem
• move pointer strategy
• container store min bound water

15. 3Sum

• List<List<Integer>> res
• skip duplicate answers outside for loop and inside for loop
• reduce three sum to two sum
• Arrays.asList(a,b,c) return List<Integer>
• O(n^2) time complexity

16. 3Sum Closest

• use two pointer strategy
• update minDiff in each loop

18. 4Sum

• use function to make code more readable
• skip duplicate
• check bound to skip or end loop

26. Remove Duplicates from Sorted Array

• confirm the return type
• in place modification of array
• move pointer strategy

27. Remove Element

• move pointer strategy
• check bound

31. Next Permutation

• find patter of permutation
• back traverse find peak, then find change location, swap change location and reverse the back part
• consider special case