dsa7 min read
Minimum Time to Collect All Apples in a Tree — LC 1443 Deep Dive
Find the minimum seconds to collect all apples in an undirected tree using post-order DFS — include a subtree path only when it contains at least one apple.
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dsa7 min read
Maximum Difference Between Node and Ancestor — LC 1026 Deep Dive
Find the maximum |a - b| for any ancestor-descendant pair by tracking the running minimum and maximum along every root-to-leaf path during DFS.
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dsa8 min read
Longest ZigZag Path in a Binary Tree — LC 1372 Deep Dive
Find the longest zigzag path in a binary tree by DFS — track the current direction and length, reset when the direction breaks, and update a global maximum.
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dsa6 min read
Sum of Distances in Tree — LeetCode 834 Rerooting Masterclass
LeetCode 834 Sum of Distances in Tree is a Google and Meta favorite hard problem solved in O(n) using two-pass DFS with the rerooting technique on an undirected tree.
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dsa5 min read
Unique Binary Search Trees — LeetCode 96 Catalan Numbers Decoded
LeetCode 96 Unique Binary Search Trees asks for the count of structurally unique BSTs storing 1..n. Solve it in O(n^2) using Catalan number DP — a favorite Amazon and Google interview question.
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