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Given the following code snippet implementing the prefix sum DFS approach for Path Sum III, what is the output for the tree [10,5,-3,3,2,null,11,3,-2,null,1] and targetSum = 8?

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Tree: Depth-First Search - Path Sum III (Any Path)
Given the following code snippet implementing the prefix sum DFS approach for Path Sum III, what is the output for the tree [10,5,-3,3,2,null,11,3,-2,null,1] and targetSum = 8? ```python class TreeNode: def __init__(self, val=0, left=None, right=None): self.val = val self.left = left self.right = right class Solution: def pathSum(self, root, targetSum): prefix_counts = {0: 1} self.result = 0 def dfs(node, current_sum): if not node: return current_sum += node.val self.result += prefix_counts.get(current_sum - targetSum, 0) prefix_counts[current_sum] = prefix_counts.get(current_sum, 0) + 1 dfs(node.left, current_sum) dfs(node.right, current_sum) prefix_counts[current_sum] -= 1 dfs(root, 0) return self.result ```
A5
B4
C3
D6
Step-by-Step Solution
Solution:
  1. Step 1: Trace prefix sums along paths

    Paths summing to 8 are: 5->3, 5->2->1, -3->11, 10->-3->11, and 3->5 (if any). Total 5 paths.
  2. Step 2: Confirm prefix_counts updates and decrements

    Prefix sums correctly count these paths without double counting.
  3. Final Answer:

    Option A -> Option A
  4. Quick Check:

    Known example from LeetCode Path Sum III returns 5 [OK]
Quick Trick: Trace prefix sums carefully to count valid paths [OK]
Common Mistakes:
MISTAKES
  • Counting only root-starting paths
  • Forgetting to decrement prefix_counts after recursion
Trap Explanation:
PITFALL
  • Candidates often overcount or undercount paths by mismanaging prefix_counts or ignoring non-root paths.
Interviewer Note:
CONTEXT
  • Tests ability to mentally execute prefix sum DFS on a canonical example.
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