beginner
11 min read
Add Binary
You receive two binary strings, a and b, and must return their sum as another binary string. The inputs contain only '0' and '1', have lengths from 1 to…
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Beginner
Articles whose authoritative difficulty is beginner.
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Beginner articles
30 articles in this tag.
beginner
9 min read
Balanced Binary Tree
A reliable Balanced Binary Tree solution carries two facts upward from every subtree: its height and whether it is balanced.
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beginner
10 min read
Binary Tree Inorder Traversal
The difficult part of inorder traversal is not remembering “left, node, right.” It is preserving the parent node while the left subtree is still…
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beginner
9 min read
Climbing Stairs
The reliable way to solve Climbing Stairs is to stop guessing “Fibonacci” and ask one structural question: what could the final move have been?
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beginner
9 min read
Convert Sorted Array to Binary Search Tree
Choosing the first or last value as the root preserves sorted order, but it produces a one-sided chain. The key is to choose a root that splits the…
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beginner
10 min read
Find the Index of the First Occurrence in a String
The code is short. The contract is precise: test every legal starting position, require a complete match, and return as soon as the earliest one succeeds.
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beginner
10 min read
Integer to Roman
The code is short. The trap is assuming the rules are short enough for a pile of special cases.
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beginner
9 min read
Length of Last Word
The final character may be a space, so the right edge of the string is not necessarily part of the answer. The clean Length of Last Word solution is a…
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beginner
10 min read
Letter Combinations of a Phone Number
Backtracking becomes much easier when you can name what one recursive call means. Here, each call assigns one phone-keypad digit, and each complete path…
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beginner
9 min read
Longest Common Prefix
The trap is to compare whole strings before identifying the stopping condition. The answer ends at the first column where agreement breaks—or when the…
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beginner
9 min read
Maximum Depth of Binary Tree
The cleanest way to solve this problem is to stop counting depth globally. Ask each subtree for its answer, then let the parent combine those answers.
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beginner
8 min read
Merge Sorted Array
A left-to-right merge can overwrite values in nums1 before you have compared them. The reliable Merge Sorted Array solution uses backward two pointers:…
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beginner
10 min read
Merge Two Sorted Lists
The common failure mode here is treating linked lists like arrays: copy the values, sort them, and rebuild. That throws away the structure the problem…
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beginner
10 min read
Minimum Depth of Binary Tree
The shortest path counts only when it reaches a leaf. That one condition is what breaks the tempting min(left, right) solution.
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beginner
10 min read
Palindrome Number
A full reverse is easy to write. The interview-level move is to stop reversing when the two halves meet.
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beginner
9 min read
Pascal's Triangle II
The requested output is one row, not the entire triangle. Build that row with one working list, and update it from right to left so each calculation still…
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beginner
10 min read
Pascal's Triangle
The output looks like mathematics. The interview task is simpler: build each row from the row you already have.
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beginner
10 min read
Path Sum
A matching sum at an internal node is not enough. The path must start at the root, end at a leaf, and satisfy the target at that boundary.
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beginner
9 min read
Plus One
Adding one looks trivial until the last digit is 9. Then the problem becomes a compact state-tracking exercise: start where arithmetic begins, propagate a…
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beginner
9 min read
Remove Duplicates from Sorted Array
The word “remove” is misleading here. You do not need to shrink the Python list or delete values from its tail. You need to compact the distinct values…
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beginner
9 min read
Remove Duplicates from Sorted List
When a problem says “remove duplicates,” the first instinct is often to reach for a set. That works for an unsorted list, but it misses the key clue here:…
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beginner
8 min read
Remove Element
The array is not shortened. The judge inspects only a prefix, so the job is to compact the values you keep into that prefix and return its length.
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beginner
10 min read
Roman to Integer
Adding every Roman symbol works for LVIII, but it turns IV into 6. The reliable Roman to Integer solution is a left-to-right scan with one local question:…
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beginner
10 min read
Same Tree
A tree comparison fails the moment you forget that a missing child is also part of the structure.
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beginner
10 min read
Search Insert Position
This problem asks for more. If the target is absent, return the index where it could be inserted while keeping the array sorted.
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beginner
10 min read
Sqrt(x)
The fastest way to solve this problem is to stop searching for a square root and search for the last integer that is still feasible.
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beginner
10 min read
Subsets
The recursive code for subsets is short. The reasoning behind it is the part worth learning: every input element creates one independent choice—include it…
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beginner
10 min read
Symmetric Tree
The key to the Symmetric Tree solution is to compare two nodes as a mirror pair, not to traverse the left and right subtrees independently.
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beginner
7 min read
Two Sum
A strong Two Sum solution replaces repeated pair scanning with one sharper question: has the array already shown us the value this number needs?
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beginner
9 min read
Valid Parentheses
Counting brackets tells you how many openings and closings exist. A stack tells you whether they close in the only order that nesting allows.
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