Day 62: Python Longest Subarray with Target Sum - O(n) Prefix Sum & HashMap Guide (LeetCode Vibes)

💡 Key Takeaways from Day 62: Longest Subarray Function ## 1. Function Design: Prefix Dict and Best Vars Setup ## 2. Loop Processing: Prefix Update and Complement Lookup ## 3. Main Interactive: Input Parsing and Subarray Output ## 🎯 Summary and Reflections ## 🚀 Next Steps and Resources Welcome to Day 62 of the #80DaysOfChallenges journey! This intermediate challenge solves the longest subarray with exact target sum using the powerhouse combo of prefix sums and a hashmap for O(n) time, finding not just any subarray but the longest one that sums to target. It's the classic "subarray sum equals k" variant (LeetCode #560 with length twist), storing earliest prefix indices to maximize window size when a match is found. If you're leveling up from basic arrays to hash-optimized algorithms, or crushing interview problems like finding max length subarrays, this "Python longest subarray target sum" script is efficient, returns length/start/end, and perfect for extensions like multiple targets or negative numbers. This task features a function that computes prefix sums on the fly, uses a dict to store earliest indices for each prefix, and updates best length on matches. It's a prefix-hash pattern: track cumulative sums, look for complement (prefix - target). We'll detail: function with prefix dict and best trackers, loop for sum update and match check, and main with input and subarray print. The longest_subarray_with_target function initializes for tracking: Dict starts with 0:-1 for subarrays from start. Vars track max length and indices. Handles negatives fine. Core enumeration scans: Adds val to prefix, stores if new. Looks for need = prefix - target, computes length if found, updates best if longer. Earliest index maximizes length. O(n) time, O(n) space. Script reads nums and target: Parses, calls, prints length/subarray or none. Handles empty gracefully. This longest target sum subarray uses prefix-hash to find max length efficiently. It reinforced: Reflections: Handles negatives/zeros, assumes majority exists not needed. For multiples, track all indices. Advanced Alternatives: Sliding window for positives. Kadane variant for max sum. Your subarray tip? Share! Day 62 unlocked subarray sums. In #80DaysOfChallenges? Found huge? Post! Templates let you quickly answer FAQs or store snippets for re-use. Are you sure you want to ? It will become hidden in your post, but will still be visible via the comment's permalink. as well , this person and/or COMMAND_BLOCK: def longest_subarray_with_target(nums: list[int], target: int) -> tuple: """ Return (length, start_idx, end_idx) of the longest subarray with sum == target. If no such subarray exists, returns (0, -1, -1). """ # prefix sum -> earliest index (sum zero before start) prefix_index = {0: -1} prefix = 0 # running prefix sum best_len = 0 # best length found so far best_start = -1 # start index of best subarray best_end = -1 # end index (inclusive) of best subarray COMMAND_BLOCK: def longest_subarray_with_target(nums: list[int], target: int) -> tuple: """ Return (length, start_idx, end_idx) of the longest subarray with sum == target. If no such subarray exists, returns (0, -1, -1). """ # prefix sum -> earliest index (sum zero before start) prefix_index = {0: -1} prefix = 0 # running prefix sum best_len = 0 # best length found so far best_start = -1 # start index of best subarray best_end = -1 # end index (inclusive) of best subarray COMMAND_BLOCK: def longest_subarray_with_target(nums: list[int], target: int) -> tuple: """ Return (length, start_idx, end_idx) of the longest subarray with sum == target. If no such subarray exists, returns (0, -1, -1). """ # prefix sum -> earliest index (sum zero before start) prefix_index = {0: -1} prefix = 0 # running prefix sum best_len = 0 # best length found so far best_start = -1 # start index of best subarray best_end = -1 # end index (inclusive) of best subarray COMMAND_BLOCK: for i, val in enumerate(nums): prefix += val # update running sum with current value # If we've never seen this prefix before, record its earliest index if prefix not in prefix_index: prefix_index[prefix] = i # Check if there's a prefix that makes [j+1 .. i] sum == target need = prefix - target if need in prefix_index: start_idx = prefix_index[need] + 1 curr_len = i - prefix_index[need] if curr_len > best_len: # update best if longer found best_len = curr_len best_start = start_idx best_end = i return best_len, best_start, best_end COMMAND_BLOCK: for i, val in enumerate(nums): prefix += val # update running sum with current value # If we've never seen this prefix before, record its earliest index if prefix not in prefix_index: prefix_index[prefix] = i # Check if there's a prefix that makes [j+1 .. i] sum == target need = prefix - target if need in prefix_index: start_idx = prefix_index[need] + 1 curr_len = i - prefix_index[need] if curr_len > best_len: # update best if longer found best_len = curr_len best_start = start_idx best_end = i return best_len, best_start, best_end COMMAND_BLOCK: for i, val in enumerate(nums): prefix += val # update running sum with current value # If we've never seen this prefix before, record its earliest index if prefix not in prefix_index: prefix_index[prefix] = i # Check if there's a prefix that makes [j+1 .. i] sum == target need = prefix - target if need in prefix_index: start_idx = prefix_index[need] + 1 curr_len = i - prefix_index[need] if curr_len > best_len: # update best if longer found best_len = curr_len best_start = start_idx best_end = i return best_len, best_start, best_end CODE_BLOCK: raw = input("Enter numbers (space-separated): ").strip() nums = list(map(int, raw.split())) t_raw = input("Enter target sum: ").strip() target = int(t_raw) length, s, e = longest_subarray_with_target(nums, target) if length == 0: print(f"No subarray sums to {target}.") else: sub = nums[s:e+1] print(f"Longest length: {length}") print(f"Subarray (indices {s}..{e}): {sub}") CODE_BLOCK: raw = input("Enter numbers (space-separated): ").strip() nums = list(map(int, raw.split())) t_raw = input("Enter target sum: ").strip() target = int(t_raw) length, s, e = longest_subarray_with_target(nums, target) if length == 0: print(f"No subarray sums to {target}.") else: sub = nums[s:e+1] print(f"Longest length: {length}") print(f"Subarray (indices {s}..{e}): {sub}") CODE_BLOCK: raw = input("Enter numbers (space-separated): ").strip() nums = list(map(int, raw.split())) t_raw = input("Enter target sum: ").strip() target = int(t_raw) length, s, e = longest_subarray_with_target(nums, target) if length == 0: print(f"No subarray sums to {target}.") else: sub = nums[s:e+1] print(f"Longest length: {length}") print(f"Subarray (indices {s}..{e}): {sub}") - Prefix sum power: Enables constant-time range sums. - Earliest index: Key for max length, not any match. - Hashmap optimization: O(n) for what brute O(n^2) does. - Source Code for Challenge #62: scripts/longest_subarray_target_sum.py - Main Repository: 80-days-of-challenges - Daily Updates: Twitter/X (@Shahrouzlogs)