daifu

/*
Given a collection of numbers that might contain duplicates, return all possible unique permutations.

For example,
[1,1,2] have the following unique permutations:
[1,1,2], [1,2,1], and [2,1,1].

Algorithm:
Basically it is similar to Permutation, but it needs
to check the condition that if the current num is the

daifu

/*
Given a collection of numbers, return all possible permutations.

For example,
[1,2,3] have the following permutations:
[1,2,3], [1,3,2], [2,1,3], [2,3,1], [3,1,2], and [3,2,1].

Algorithm:
1. check if the list is equal to the size of the num array
2. loop through all the possibility

daifu

/*
You are given an n x n 2D matrix representing an image.

Rotate the image by 90 degrees (clockwise).

Follow up:
Could you do this in-place?

Basic approach has 2 steps:
1. rotate based on the right most to left bottom diagonal

daifu

/*
Cases need to be taken care of:
1. sign
2. extra space in the front
3. half valid number
4. overflow for the large number
5. invalid number
*/
public class Solution {
    public int atoi(String str) {

daifu

/*
Given preorder and inorder traversal of a tree, construct the binary tree.

Note:
You may assume that duplicates do not exist in the tree.
*/
/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;

daifu

/*
The gray code is a binary numeral system where two successive values differ in only one bit.

Given a non-negative integer n representing the total number of bits in the code, print the sequence of gray code. A gray code sequence must begin with 0.

For example, given n = 2, return [0,1,3,2]. Its gray code sequence is:

00 - 0
01 - 1
11 - 3

daifu

/*
A message containing letters from A-Z is being encoded to numbers using the following mapping:

'A' -> 1
'B' -> 2
...
'Z' -> 26
Given an encoded message containing digits, determine the total number of ways to decode it.

For example,

daifu

/*
Given an index k, return the kth row of the Pascal's triangle.

For example, given k = 3,
Return [1,3,3,1].

Note:
Could you optimize your algorithm to use only O(k) extra space?
*/
public class Solution {

daifu

/*
Given numRows, generate the first numRows of Pascal's triangle.

For example, given numRows = 5,
Return

[
     [1],
    [1,1],
   [1,2,1],

daifu

/*
Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

For example,
If n = 4 and k = 2, a solution is:

[
  [2,4],
  [3,4],
  [2,3],