Leetcode

Subtitle / Abstract A basic counting problem: use frequency + combinations to drop O(n^2) to O(n). Includes engineering use cases and portable implementations. Reading time: 8-10 minutes Tags: hash-table, counting, array SEO keywords: Good Pairs, hash map, frequency Meta description: Hash counting solution for Good Pairs with complexity and code. Target readers Beginners learning hash tables and counting Engineers who want to map interview patterns to real stats tasks Interview prep for basic counting models Background / Motivation Counting equal pairs is a classic problem. A double loop is O(n^2). With frequency counting, you can solve it in linear time and scale to large data. ...

LeetCode 1: Two Sum Summary Find two indices such that nums[i] + nums[j] = target. Use a hash map for O(n) time. Approach Iterate and store value -> index. For each number x, check if target - x exists. Complexity Time: O(n) Space: O(n) Python reference implementation def two_sum(nums, target): seen = {} for i, x in enumerate(nums): y = target - x if y in seen: return [seen[y], i] seen[x] = i

LeetCode 2300: Successful Pairs of Spells and Potions Summary For each spell, count how many potions make spell * potion >= success. Sort potions and binary search the threshold. Approach Sort potions. For each spell, compute need = ceil(success / spell). Use binary search to find the first potion >= need. Complexity Time: O(n log n) Space: O(1) extra (or O(n) if sorting a copy) Python reference implementation import bisect import math def successful_pairs(spells, potions, success): potions = sorted(potions) n = len(potions) res = [] for s in spells: need = (success + s - 1) // s idx = bisect.bisect_left(potions, need) res.append(n - idx) return res

LeetCode 2379: Minimum Recolors to Get K Consecutive Black Blocks Summary Given a string of ‘B’ and ‘W’, find the minimum recolors to make a substring of length k all black. Approach Use a sliding window of length k and count the number of whites in the window. The minimum whites across all windows is the answer. Complexity Time: O(n) Space: O(1) Python reference implementation def minimum_recolors(blocks, k): whites = sum(1 for c in blocks[:k] if c == 'W') ans = whites for i in range(k, len(blocks)): if blocks[i-k] == 'W': whites -= 1 if blocks[i] == 'W': whites += 1 ans = min(ans, whites) return ans

LeetCode 2841: Maximum Sum of Almost Unique Subarray Summary Given an array, window size k, and threshold m, find the maximum sum of any length-k subarray that contains at least m distinct elements. Approach Use a sliding window with a frequency map, track window sum and number of distinct values. Complexity Time: O(n) Space: O(n) for frequency map Python reference implementation def max_sum_almost_unique(nums, m, k): from collections import defaultdict count = defaultdict(int) distinct = 0 window_sum = 0 ans = 0 for i, x in enumerate(nums): window_sum += x if count[x] == 0: distinct += 1 count[x] += 1 if i >= k: y = nums[i - k] window_sum -= y count[y] -= 1 if count[y] == 0: distinct -= 1 if i >= k - 1 and distinct >= m: ans = max(ans, window_sum) return ans