Hashing & Maps — Complete Guide (Problems 161–205)

# Complement Map (Two Sum)
def two_sum(nums, target):
    seen = {}
    for i, x in enumerate(nums):
        if target - x in seen: return [seen[target-x], i]
        seen[x] = i

# Frequency Counter
from collections import Counter
def freq_map(s): return Counter(s)

# Prefix Sum + HashMap
from collections import defaultdict
def subarray_sum_k(nums, k):
    prefix = cnt = 0
    mp = defaultdict(int); mp[0] = 1
    for n in nums:
        prefix += n
        cnt += mp[prefix - k]
        mp[prefix] += 1
    return cnt

# LRU Cache (OrderedDict)
from collections import OrderedDict
class LRUCache:
    def __init__(self, cap):
        self.cap = cap; self.cache = OrderedDict()
    def get(self, key):
        if key not in self.cache: return -1
        self.cache.move_to_end(key); return self.cache[key]
    def put(self, key, val):
        if key in self.cache: self.cache.move_to_end(key)
        self.cache[key] = val
        if len(self.cache) > self.cap: self.cache.popitem(last=False)