Day 18 of LeetCode

Documenting LeetCode solving.

Q54

Medium. Tree

BFS

class Solution:
    def rightSideView(self, root: Optional[TreeNode]) -> List[int]:
        res = []
        q = collections.deque()
        q.append(root)

        while q:
            rightSide = None
            qLen = len(q)

            for i in range(qLen):
                node = q.popleft()
                if node:
                    rightSide = node
                    q.append(node.left)
                    q.append(node.right)

            if rightSide:
                res.append(rightSide.val)

        return res

Q55

1448. Count Good Nodes in Binary Tree

Medium. Tree

Preorder traverse.

class Solution:
    def goodNodes(self, root: TreeNode) -> int:

        def dfs(node, maxVal):
            if not node:
                return 0
            # Preorder
            res = 1 if node.val >= maxVal else 0
            maxVal = max(maxVal, node.val)
            res += dfs(node.left, maxVal)
            res += dfs(node.right, maxVal)

            return res

        return dfs(root, root.val)

Q56

98. Validate Binary Search Tree

Medium. Tree

Keep track of the left and right boundary.

class Solution:
    def isValidBST(self, root: Optional[TreeNode]) -> bool:

        def valid(node, left, right):
            if not node:
                return True
            if not (node.val > left and node.val < right):
                return False

            return (valid(node.left, left, node.val) and
                    valid(node.right, node.val, right))

        return valid(root, float("-infinity"), float("infinity"))