Leetcode for fun

One question a day to ensure a sharp mind.

Grading Criteria

• L0: straight forward question
• L1: variance of template
• L2: need to think for a while / complex implementation
• L3: need aha moment / unexpected algorithm

Time Complexity Analysis

Row: input size(IS), column: time complexity(TC)

IS&TC	O($2^n$)	O($n^4$)	O($n^3$)	O($n^2$)	O(nlogn)	O(n)	O(logn)	O(1)
1-10	✓	✓	✓	✓	✓	✓	✓	✓
10-50	✓	✓	✓	✓	✓	✓	✓	✓
50-100	✗	✗	✓	✓	✓	✓	✓	✓
100-500	✗	✗	✓	✓	✓	✓	✓	✓
500 - ${10}^3$	✗	✗	✗	✓	✓	✓	✓	✓
${10}^3$ - ${10}^4$	✗	✗	✗	✓	✓	✓	✓	✓
${10}^4$ - ${10}^5$	✗	✗	✗	?	✓	✓	✓	✓
${10}^5$ - ${10}^6$	✗	✗	✗	✗	✓	✓	✓	✓
${10}^6$ - ${10}^9$	✗	✗	✗	✗	✗	✗	✓	✓

TC	Algorithm
O($2^n$)	DFS-combination($2^n$), DFS-permutation(n!),
O($n^4$)	DP
O($n^3$)	DP, Floyd-Warshall
O($n^2$)	DP
O(nlogn)	Sorting, Heap, divide&conquer, Dijkstra-heap, QuickSort
O(n)	DP, DFS-tree(V), BFS(V+E), TopologicalSorting(V+E), BucketSort(N+K), MonotonicStack()
O(logn)	BinarySearch, BinaryIndexTree
O(1)	Math

Trigger keywords

1. What is the data size?
2. Can I sort or group the elements?
3. Can I use DP, greedy or binary search?
4. Can I enumerate on specific variable?
5. Can I use two passes?
6. Can I solve it reversely?
7. Can I convert the problem to a other problem?
8. If the problem mentions "Subarray", consider sliding window, monotonic stack/queue, prefix sum + hash table, Kadane's algorithm.

Cheat sheet

Palindrome

Efficiently find all the palindrome numbers in a range 10**9:

pal = []
base = 1
while base <= 10000: # 
    # odd number
    for i in range(base, base * 10):
        x = i
        t = i // 10
        while t:
            x = x * 10 + t % 10
            t //= 10
        pal.append(x)
    # even number
    if base <= 1000:
        for i in range(base, base * 10):
            x = t = i
            while t:
                x = x * 10 + t % 10
                t //= 10
            pal.append(x)
    base *= 10
pal.append(1_000_000_001)  # sentinel

Reference

1. 用什么语言刷题？C++/Java/Python横向大比较
2. Leetcode 101: A Leetcode Grinding Guide(C++ Version)
3. Algorithms for Competitive Programming
4. 古城算法 slides(google drive)
5. 输入数据规模和时间复杂度的关系
6. 0x3ff-palindrome