LeetCode

Foreword

LeetCode is a cool platform for practicing programming as problems described here are rudimental properly compared to ones in CodeForce, POJ, etc and the user experience is excellent in contrast.

The deduction of solution is more important than the solution itself especially when you work it out for self-improvement. The solution to problem in a small scale is always easy to come up with, say, brute force, while it's bloody difficult in large scale. TLE, Stack Overflow, etc, pop up in your way to be Accepted. Complexity of time and space is the critical concept in algorithm design which you should always keep in your mind. Think in large scale.

Retrospect

Sometimes the most straight-forward solutions to some questions are incredibly simple. For example, "Binary Tree Postorder Traversal". A brain-dead recursion may actually be accepted. Cost of time is O(n). Good!!! Cost of space is O(n). Acceptable!!! ... Come on, don't be such a baby. If so, you can't get any improvement after solving this "stupid" question. Try an iterative way with the same complexity of both time and space. Then try a new way with just O(1) space known as “Morris Traversal”...

Whenever you meet a linked-list-related problem, dummy head should be come up in your mind in conditioned response. It's so helpful to have a "useless" node pointing to the real head that you can find out some solutions accidentally at times and make your code more tidy empirically. Of course it's unnecessary to add such an extra character in some situation (say, traveling a linked list, maximum node in a linked list, whatever).

DFS and BFS should always be at the head of your chain of thought when you meet a question not quite complicated literally or brute-force-enabled. It's straight-forward and non-tricky. Moreover, it works.

Dynamic-programming-related problems given in LeetCode (say, Triangle, Edit Distance, etc), are too few and mildly fundamental. Frankly, dynamic programming plays a crucial role in algorithm design. If you really want to dig out something internal and heuristic in rationale, don't be satisfied by LeetCode.

When you find all pairs can be generated from the multiplication of all elements in left subtree and in right subtree respectively, a pitfall may show up. When you find some elements in the result set happen to be duplicated, that means the idea your solution based on was almost incorrect. There is no need to dig out any tricks from it anymore. Try a simple DFS or BFS. You may see the light.

Every recursive solution could be converted to an iterative one deliberately considering that there is no real recursive logic in the process of your final executable program actually. Think about how your computer works when you call a function so that you could think it through. It is roughly as:

Put parameter variables on stack.
Put local variables on stack.
Jump to the address of the function.
Load variables from stack.
Run the logic in function.
clean stack.
Jump back to caller's space.

Overtly stack is the key point. If you want a conversion from recursion to iteration, stack will be the primitive bridge promising and non-tricky by which it's easy to figure a way out. Necessary is it to make such a conversion in case of large scale. Recursion is easier to come up with and more readable than iteration while correspondingly it would lay higher claim to memory. Local variables would be stacked over and over during the recursive call until the whole invoke finishes or stack overflows to crash. Problem solved when you use heap-allocated stack from STL. Anyway, each way has its merit and is needed in specific case. You have better grasp both of them.

The widespread use of stack as a LIFO associative container is not limited in just being a conversion bridge. When you try using stack to store some extra information for your solution, at first you may consider having elements in your target data set stored in it. However, if you find any of your ideas in which individual's value doesn't make a difference. It's time for you to store something else instead of the raw value. You may take a try to store the frequency of occurrence of elements visited or anything else meaningful.

Summary

A problem is there, a solution is there. Life is not easy, especially for coder. Long may you code...

Name		Name	Last commit message	Last commit date
Latest commit History 14 Commits
3sum closest.cpp		3sum closest.cpp
3sum.cpp		3sum.cpp
4sum.cpp		4sum.cpp
Add Binary.cpp		Add Binary.cpp
Add Two Numbers.cpp		Add Two Numbers.cpp
Best Time to Buy and Sell Stock II.cpp		Best Time to Buy and Sell Stock II.cpp
Best Time to Buy and Sell Stock III.cpp		Best Time to Buy and Sell Stock III.cpp
Best Time to Buy and Sell Stock.cpp		Best Time to Buy and Sell Stock.cpp
Binary Tree Inorder Traversal.cpp		Binary Tree Inorder Traversal.cpp
Binary Tree Level Order Traversal .cpp		Binary Tree Level Order Traversal .cpp
Binary Tree Level Order Traversal II.cpp		Binary Tree Level Order Traversal II.cpp
Binary Tree Maximum Path Sum.cpp		Binary Tree Maximum Path Sum.cpp
Binary Tree Postorder Traversal.cpp		Binary Tree Postorder Traversal.cpp
Binary Tree Preorder Traversal.cpp		Binary Tree Preorder Traversal.cpp
Binary Tree Zigzag Level Order Traversal.cpp		Binary Tree Zigzag Level Order Traversal.cpp
Climbing Stairs.cpp		Climbing Stairs.cpp
Clone Graph.cpp		Clone Graph.cpp
Combination Sum II.cpp		Combination Sum II.cpp
Combination Sum.cpp		Combination Sum.cpp
Combinations.cpp		Combinations.cpp
Construct Binary Tree from Inorder and Postorder Traversal.cpp		Construct Binary Tree from Inorder and Postorder Traversal.cpp
Construct Binary Tree from Preorder and Inorder Traversal.cpp		Construct Binary Tree from Preorder and Inorder Traversal.cpp
Container With Most Water.cpp		Container With Most Water.cpp
Convert Sorted Array to Binary Search Tree.cpp		Convert Sorted Array to Binary Search Tree.cpp
Convert Sorted List to Binary Search Tree.cpp		Convert Sorted List to Binary Search Tree.cpp
Copy List with Random Pointer.cpp		Copy List with Random Pointer.cpp
Count and Say.cpp		Count and Say.cpp
Edit Distance.cpp		Edit Distance.cpp
Evaluate Reverse Polish Notation.cpp		Evaluate Reverse Polish Notation.cpp
First Missing Positive.cpp		First Missing Positive.cpp
Flatten Binary Tree to Linked List.cpp		Flatten Binary Tree to Linked List.cpp
Gas Station.cpp		Gas Station.cpp
Generate Parentheses.cpp		Generate Parentheses.cpp
Implement strStr().cpp		Implement strStr().cpp
Insert Interval.cpp		Insert Interval.cpp
Insertion Sort List.cpp		Insertion Sort List.cpp
Integer to Roman.cpp		Integer to Roman.cpp
Jump Game II.cpp		Jump Game II.cpp
Jump Game.cpp		Jump Game.cpp
LRU Cache.cpp		LRU Cache.cpp
Length of Last Word.cpp		Length of Last Word.cpp
Letter Combinations of a Phone Number.cpp		Letter Combinations of a Phone Number.cpp
Linked List Cycle II.cpp		Linked List Cycle II.cpp
Linked List Cycle.cpp		Linked List Cycle.cpp
Longest Common Prefix.cpp		Longest Common Prefix.cpp
Longest Consecutive Sequence.cpp		Longest Consecutive Sequence.cpp
Longest Substring Without Repeating Characters.cpp		Longest Substring Without Repeating Characters.cpp
Longest Valid Parentheses.cpp		Longest Valid Parentheses.cpp
Max Sliding Window.cpp		Max Sliding Window.cpp
Maximum Subarray.cpp		Maximum Subarray.cpp
Merge Intervals.cpp		Merge Intervals.cpp
Merge Sorted Array.cpp		Merge Sorted Array.cpp
Merge Two Sorted Lists.cpp		Merge Two Sorted Lists.cpp
Merge k Sorted Lists.cpp		Merge k Sorted Lists.cpp
Minimum Depth of Binary Tree.cpp		Minimum Depth of Binary Tree.cpp
Minimum Path Sum.cpp		Minimum Path Sum.cpp
Minimum Window Substring.cpp		Minimum Window Substring.cpp
Multiply Strings.cpp		Multiply Strings.cpp
Palindrome Number.cpp		Palindrome Number.cpp
Palindrome Partitioning II.cpp		Palindrome Partitioning II.cpp
Palindrome Partitioning.cpp		Palindrome Partitioning.cpp
Partition List.cpp		Partition List.cpp
Pascal's Triangle II.cpp		Pascal's Triangle II.cpp
Pascal's Triangle.cpp		Pascal's Triangle.cpp
Path Sum II.cpp		Path Sum II.cpp
Path Sum.cpp		Path Sum.cpp
Permutation Sequence.cpp		Permutation Sequence.cpp
Permutations I.cpp		Permutations I.cpp
Permutations II.cpp		Permutations II.cpp
Plus One.cpp		Plus One.cpp
Populating Next Right Pointers in Each Node II.cpp		Populating Next Right Pointers in Each Node II.cpp
Populating Next Right Pointers in Each Node.cpp		Populating Next Right Pointers in Each Node.cpp
README.md		README.md
Recover Binary Search Tree.cpp		Recover Binary Search Tree.cpp
Remove Duplicates from Sorted Array II.cpp		Remove Duplicates from Sorted Array II.cpp
Remove Duplicates from Sorted Array.cpp		Remove Duplicates from Sorted Array.cpp
Remove Duplicates from Sorted List.cpp		Remove Duplicates from Sorted List.cpp
Remove Element.cpp		Remove Element.cpp
Remove Nth Node From End of List.cpp		Remove Nth Node From End of List.cpp
Reorder List.cpp		Reorder List.cpp
Reverse Integer.cpp		Reverse Integer.cpp
Reverse Linked List II.cpp		Reverse Linked List II.cpp
Reverse Words in a String.cpp		Reverse Words in a String.cpp
Roman to Integer.cpp		Roman to Integer.cpp
Rotate Image.cpp		Rotate Image.cpp
Rotate List.cpp		Rotate List.cpp
Same Tree.cpp		Same Tree.cpp
Scramble String.cpp		Scramble String.cpp
Search for a Range.cpp		Search for a Range.cpp
Search in Rotated Sorted Array.cpp		Search in Rotated Sorted Array.cpp
Set Matrix Zeroes.cpp		Set Matrix Zeroes.cpp
Single Number II.cpp		Single Number II.cpp
Single Number.cpp		Single Number.cpp
Sort Colors.cpp		Sort Colors.cpp
Sort List.cpp		Sort List.cpp
Spiral Matrix II.cpp		Spiral Matrix II.cpp
Spiral Matrix.cpp		Spiral Matrix.cpp
Subsets II.cpp		Subsets II.cpp
Subsets.cpp		Subsets.cpp
Sum Root to Leaf Numbers.cpp		Sum Root to Leaf Numbers.cpp

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