Never be negative

Print right view of binary tree

Given a binary tree, imagine yourself standing on the right side of it, return the values of the nodes you can see ordered from top to bottom.

For example:
Given the following binary tree,

   1            <---
 /   \
2     3         <---
 \     \
  5     4       <---

You should return [1, 3, 4].

Credits:
Special thanks to @amrsaqr for adding this problem and creating all test cases.

Number times

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

K sum problem

2 SUM I II III

3 SUM

3 SUM CLOEST

4 SUM

K SUM I II

One vs all solutions

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.