1 <--- / \ 2 3 <--- \ \ 5 4 <---
Single Number I, all twice except one single
Single Number II All numbers occurs triple times except one once
Single Number II All numbers occurs triple times except one twice
Single Number III all twice numbers, except two single Number
2 SUM I II III
3 SUM CLOEST
K SUM I II
This kind of question looks this way,
1, need find all possible solutions, we need backtracking method to find them.
2, only need to check if it is available or not, check.
3, only need for a optimist value, Dynamic programming would be very helpful here.
Usually we need dp for optimist solution, backtracking for all solutions, and another idea for avaliable
or statics solution.
Introduction on different version of tree traverse, first is o(n) time,o(n)time, we use a stack,
then try using o(1) space by morris tree. there is also a special method for post traverse
by using two stacks o(2n).