Understand what the interviewer is asking for by using test cases and questions about the problem.
- Established a set (2-3) of test cases to verify their own solution later.
- Established a set (1-2) of edge cases to verify their solution handles complexities.
- Have fully understood the problem and have no clarifying questions.
- Have you verified any Time/Space Constraints for this problem?
HAPPY CASE
Input: [1, 2, 3, 4, null, 5, 6, null, null, 7, 8, null, 9]
[1]
/ \
[2] [3]
/ / \
[4] [5] [6]
/ \ \
[7] [8] [9]
Output: 3 ([1] -> [2] -> [4] path has 3 nodes)
Input: root = [3,9,20,null,null,15,7]
Output: 2
EDGE CASE
Input: [6]
Output: 1 (root is a leaf, so path from root to leaf goes through 1 node)Match what this problem looks like to known categories of problems, e.g. Linked List or Dynamic Programming, and strategies or patterns in those categories.
For trees, some things we should consider are:
Plan the solution with appropriate visualizations and pseudocode.
Approach #1: Traverse the tree via DFS, keeping track of the current depth in the tree. When we find a leaf, update the minimum depth and return min depth at the end.
1) Traverse the tree via DFS
2) As we traverse, keep track of the current level in the tree, starting with level 0 for the root node
3) Every time we visit a left/right child of the current node, increment the level by 1
4) If we visit a node with no children, it is a leaf, and its depth is its level + 1. Track the depths of discovered leaves.
5) Return the minimum of all leaf depths.
Time Complexity: O(N) since we visit all nodes in the tree
Space Complexity: O(1) if tracking only the minimum depth found so far, O(N) if we store all depths
Cons:
- We have to visit every node in the tree because we are using DFS (the min-depth root-to-leaf path might be the last one we traverse)Approach #2:Traverse the tree via BFS (level-by-level), keeping track of current depth in the tree. Return depth of the first leaf node encountered.
1) Traverse the tree via BFS (using a queue)
2) As we traverse, keep track of current level in the tree, starting with level 0 for the root node
3) Once we find a leaf, return its depth
Time Complexity: O(N) since we (in the worst case) visit all nodes in the tree
Space Complexity: O(1) (not including space needed for queue)
Cons:
Have to use an auxiliary data structure (queue) to process nodes⚠️ Common Mistakes
Implement the code to solve the algorithm.
Approach #1: DFS
#Approach 1: DFS
def minDepth(self, root):
"
:type root: TreeNode
:rtype: int
"
# a tree with no nodes has min depth 0
if root == None:
return 0
# create a queue, and initialize it with the root node that has depth 1
queue = deque()
queue.append((root, 1))
# BFS traversal
while len(queue) > 0:
curr = queue.popleft()
currNode, currLevel = curr[0], curr[1]
# BFS traversals go level-by-level, so the first leaf found will be at the highest level of any leaf, and thus will have the min depth of any leaves
# therefore, we can return its level right away
if currNode.left == None and currNode.right == None:
return currLevel
# if the node is not a leaf, continue visiting its children, which are at level = currLevel + 1
if currNode.left != None:
queue.append((currNode.left, currLevel + 1))
if currNode.right != None:
queue.append((currNode.right, currLevel + 1))
return -1Approach #2: BFS
#Approach 2: BFS
def minDepth(self, root):
"
:type root: TreeNode
:rtype: int
"
# this helper function will find the depth of all leaf nodes in the tree
# if the current node is not a leaf, it will continue traversal, incrementing the current depth we are at
def minDepthHelper(node, cur_depth) -> int:
# if the node is a leaf, its min depth is the current depth
if not node.left and not node.right:
return cur_depth
# visit the left child to look for leaves, incrementing current depth by 1
left_depth = float('inf')
if node.left:
left_depth = minDepthHelper(node.left, cur_depth+1)
# visit the right child to look for leaves, incrementing current depth by 1
right_depth = float('inf')
if node.right:
right_depth = minDepthHelper(node.right, cur_depth+1)
# return the minimum leaf depth found between left paths and right paths
return min(left_depth, right_depth)
# a tree with no nodes has min depth 0
if not root:
return 0
# start traversal at the root, which has depth = 1
return minDepthHelper(root, 1)Review the code by running specific example(s) and recording values (watchlist) of your code's variables along the way.
Evaluate the performance of your algorithm and state any strong/weak or future potential work.
Assume N represents the number of nodes in the tree.
