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[Leetcode] Validate Binary Search Tree

[Leetcode] Validate Binary Search Tree

Many tree questions can be addressed using recursive (helper) functions. To keep the variation for specific variables along the recursion…
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[Leetcode] Construct Binary Tree from Preorder and Inorder Traversal

[Leetcode] Construct Binary Tree from Preorder and Inorder Traversal

The preorder and inorder list share some sequence pattern, suggesting which index/element to be constructed as root’s left and right.
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[Leetcode] Design Add and Search Words Data Structure

A divergence of trie. While some solution uses Trienode, it may be too slow and a simple dictionary could work.
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[Leetcode] Number of Islands

Classic combination of matrix and DFS / BFS. Use a matrix “visited” of same size to flag visited nodes and stop duplicate path. Values or…
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[Leetcode] Course Schedule

The condition of question leads to Cycle detection for directed graph. DFS would help.
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[Leetcode] Subtree of Another Tree

[Leetcode] Subtree of Another Tree

Extension of same tree. Practice template of DFS in tree.
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[Leetcode] Implement Trie (Prefix Tree)

Practice template of DFS in tree. The interesting difference is the trie permits multiple decendants rather than two in same level.
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[Leetcode] Kth Smallest Element in a BST

[Leetcode] Kth Smallest Element in a BST

Simpler than first impression. Inorder traversal makes a sorted array in BST. Stop as the k element is done.
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[Leetcode] Binary Tree Level Order Traversal

[Leetcode] Binary Tree Level Order Traversal

A must-practiced question of BFS.
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[Leetcode] Pacific Atlantic Water Flow

[Leetcode] Pacific Atlantic Water Flow

A more complicated version of Number of Islands.
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