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// node.left -> node.right -> node

public void postorder(TreeNode root) {
    
    Stack<Integer> stack1 = new Stack<>();
    Stack<Integer> stack2 = new Stack<>();
    
    stack1.push(root);
    
    TreeNode current;

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// node.left -> node -> node.right

public void inorder(TreeNode root) {
    Stack<Integer> stack = new Stack<>();
    
    TreeNode current = root;
    
    while (!stack.empty() || current != null) {
        if (current != null) {
            stack.push(current);

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# Preorder traversal: node -> node.left -> node.right
public void preorder(TreeNode root) {
    
    Stack<TreeNode> stack = new Stack<>();
    
    TreeNode current = root;
    
    while (stack.size > 0 || current != null) {
        if (current != null) {
            visit(current);

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Notes

.       - Any Character Except New Line
\d      - Digit (0-9)
\D      - Not a Digit (0-9)
\w      - Word Character (a-z, A-Z, 0-9, _)
\W      - Not a Word Character
\s - Whitespace (space, tab, newline)

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### Lambda

Syntax: ```lambda arguments: expression```

__Example__
```
# Regular way of creating a function
def add(x, y):
    return x+y
    

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Notes

Option 1: Using GitHub merge pull request button

Option 2: Execute followings steps

1. Add requester's fork as a remote
2. git fetch <remote>
3. git merge <remote>/master
4. git push

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Connect the local repo to the remote one

git remote add <name> <url>

git push -u origin master

# display all remote repos that currently connected with
git remote -v

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```
git commit -m "title" -m "longer descriptions"

# or

git commit # an editor will be opened and you will be able to type your commit title and descriptions there
```

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```
git config --global core.editor vim|emacs|nano

# Add flag --wait for your favorite editor in order to tell git 
# to wait until I quit atom for the action to be completed
```

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```
# Add the missing files from the previous commit
git add missing-file.txt

# Modify the last commit with new file added and/or new commit message
git commit --amend
```

`git commit --amend` can only be used when you forgot files from the last commit.