Day 11

Binary Tree Preorder Traversal

class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        preorder(root, res);
        return res;
    }

    public void preorder(TreeNode node, List<Integer> res) {
        if (node == null) {
            return;
        } 
        res.add(node.val);
        preorder(node.left, res);
        preorder(node.right, res);
    }
}

func preorderTraversal(root *TreeNode) []int {
	var res []int
	var traversal func(node *TreeNode)
	traversal = func(node *TreeNode) {
		if node == nil {
			return
		}
		res = append(res, node.Val)
		traversal(node.Left)
		traversal(node.Right)
	}
	traversal(root)
	return res
}

Binary Tree Postorder Traversal

class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        postOrder(root, res);
        return res;
    }

    public void postOrder(TreeNode node, List<Integer> res) {
        if (node == null) {
            return;
        }

        postOrder(node.left, res);
        postOrder(node.right, res);
        res.add(node.val);
    }
}

func postorderTraversal(root *TreeNode) []int {
	var res []int
	var traversal func(node *TreeNode)
	traversal = func(node *TreeNode) {
		if node == nil {
			return
		}
		traversal(node.Left)
		traversal(node.Right)
		res = append(res, node.Val)
	}
	traversal(root)
	return res
}

Binary Tree Inorder Traversal

class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        inOrder(root, res);
        return res;
    }

    public void inOrder(TreeNode node, List<Integer> res) {
        if (node == null) {
            return;
        }

        inOrder(node.left, res);
        res.add(node.val);
        inOrder(node.right, res);
    }
}

func inorderTraversal(root *TreeNode) []int {
	var res []int
	var traversal func(node *TreeNode)
	traversal = func(node *TreeNode) {
		if node == nil {
			return
		}
		traversal(node.Left)
		res = append(res, node.Val)
		traversal(node.Right)
	}
	traversal(root)
	return res
}

Binary Tree Level Order Traversal

class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        if (root == null) {
            return res;
        }
        
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);

        while (!queue.isEmpty()) {
            List<Integer> level = new ArrayList<>();
            int size = queue.size();
            while (size > 0) {
                TreeNode node = queue.poll();
                level.add(node.val);
                if (node.left != null) {
                    queue.offer(node.left);
                }
                if (node.right != null) {
                    queue.offer(node.right);
                }
                size--;
            }
            res.add(level);
        }
        return res;
    }
}

func levelOrder(root *TreeNode) [][]int {
	res := make([][]int, 0)
	if root == nil {
		return res
	}

	queue := make([]*TreeNode, 0)
	queue = append(queue, root)

	for len(queue) > 0 {
		size := len(queue)
		level := make([]int, 0)
		for i := 0; i < size; i++ {
			node := queue[0]
			queue = queue[1:]
			level = append(level, node.Val)
			if node.Left != nil {
				queue = append(queue, node.Left)
			}
			if node.Right != nil {
				queue = append(queue, node.Right)
			}
		}
		res = append(res, level)
	}
	return res
}