# Leetcode: Binary Tree Inorder Traversal

Given a binary tree, return the inorder traversal of its nodes’ values.

For example:
Given binary tree `{1,#,2,3}`,

```   1
\
2
/
3```

return `[1,3,2]`.

1. Recursive Version

```public ArrayList<Integer> inorderTraversal(TreeNode root) {
//recursive version
ArrayList<Integer> result = new ArrayList<>();
traversalHelper(root, result);
return result;
}

public void traversalHelper(TreeNode root, ArrayList<Integer> result) {
if (root == null) {
return;
}
traversalHelper(root.left, result);
traversalHelper(root.right, result);
}```

2. Non-recursive Version

```public ArrayList<Integer> inorderTraversal(TreeNode root) {
//non-recursive version
ArrayList<Integer> result = new ArrayList<>();
Stack<TreeNode> stack = new Stack<>();
TreeNode curr = root;
while (curr != null || !stack.empty()) {
TreeNode left = curr;
while (left != null) {
stack.push(left);
left = left.left;
}
curr = stack.pop();
curr = curr.right;
}
return result;
}```
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# Leetcode: Anagrams

Given an array of strings, return all groups of strings that are anagrams.

Note: All inputs will be in lower-case.

```public ArrayList<String> anagrams(String[] strs) {
// Start typing your Java solution below
// DO NOT write main() function
ArrayList<String> output = new ArrayList<String>();
HashMap<String,ArrayList<String>> patterns = new HashMap<String,ArrayList<String>>();

for(int i=0;i<strs.length;i++){
char[] charArr = strs[i].toCharArray();
Arrays.sort(charArr);
String sorted = new String(charArr);

if(patterns.containsKey(sorted)){
}else{
ArrayList<String> newPattern = new ArrayList<String>();
patterns.put(sorted,newPattern);
}
}

for(String str :patterns.keySet()){
if(patterns.get(str).size()>1){
}
}
return output;
}```

Analysis: Time complexity: O(n*mlgm)  (n is # of strings, m is the average length of each string) Space Complexity: O(n*m)

You are given two linked lists representing two non-negative numbers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8

```/**
* public class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) {
*         val = x;
*         next = null;
*     }
* }
*/
public class Solution {
public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
// Start typing your Java solution below
// DO NOT write main() function

int carry = 0;
while(l1!=null || l2!=null){
int val1 = l1==null?0:l1.val;
int val2 = l2==null?0:l2.val;
int sum = val1 + val2 + carry;

if(sum>9){
sum = sum - 10;
carry = 1;
}else{
carry = 0;
}
}else{
ListNode newNode = new ListNode(sum);
runner.next = newNode;
runner = runner.next;
}
l1 = l1==null?null:l1.next;
l2 = l2==null?null:l2.next;
}

if(carry==1){
ListNode newNode = new ListNode(1);
runner.next = newNode;
}
}
}```

# Leetcode: 3Sum

Given an array S of n integers, are there elements abc in S such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

Note:

• Elements in a triplet (a,b,c) must be in non-descending order. (ie, a ≤ b ≤ c)
• The solution set must not contain duplicate triplets.
```    For example, given array S = {-1 0 1 2 -1 -4},

A solution set is:
(-1, 0, 1)
(-1, -1, 2)```
```public class Solution {
public ArrayList<ArrayList<Integer>> threeSum(int[] num) {
// Start typing your Java solution below
// DO NOT write main() function
ArrayList<ArrayList<Integer>> output = new ArrayList<ArrayList<Integer>>();
if(num==null||num.length<3){
return output;
}
HashSet<ArrayList<Integer>> pairsFound = new HashSet<ArrayList<Integer>>();
Arrays.sort(num);
for(int i=0;i<num.length;i++){
ArrayList<ArrayList<Integer>> result = twoSum(num, 0-num[i]);
for(ArrayList<Integer> pair:result){
if(pair.get(0)!=i && pair.get(1)!=i){
int[] temp = new int[3];
temp[0] = i;
temp[1] = pair.get(0);
temp[2] = pair.get(1);
Arrays.sort(temp);
ArrayList<Integer> newTriple = new ArrayList<Integer>();
for(int j=0;j<temp.length;j++){
}
if(!pairsFound.contains(newTriple)){
}
}
}
}
return output;
}

public ArrayList<ArrayList<Integer>> twoSum(int[] num, int sum){
ArrayList<ArrayList<Integer>> output = new ArrayList<ArrayList<Integer>>();
int i = 0;
int j = num.length-1;
while(i<j){
if(num[i]+num[j]==sum){
ArrayList<Integer> newPair = new ArrayList<Integer>();