Graphs: visit once, then track prerequisites

"""Reference solutions. Try the exercises in README.md before opening this file."""
from collections import Counter, OrderedDict, deque
from heapq import heappop, heappush, nlargest


def two_sum(nums, target):
    visited = {}
    for j, value in enumerate(nums):
        complement = target - value
        if complement in visited:
            return visited[complement], j
        visited.setdefault(value, j)
    return None


def longest_unique(text):
    left = best = 0
    last_seen = {}
    for right, char in enumerate(text):
        left = max(left, last_seen.get(char, -1) + 1)
        best = max(best, right - left + 1)
        last_seen[char] = right
    return best


def subarray_sum(nums, target):
    prefix_counts = {0: 1}
    prefix = result = 0
    for value in nums:
        prefix += value
        result += prefix_counts.get(prefix - target, 0)
        prefix_counts[prefix] = prefix_counts.get(prefix, 0) + 1
    return result


def lower_bound(nums, target):
    lo, hi = 0, len(nums)
    while lo < hi:
        mid = lo + (hi - lo) // 2
        if nums[mid] < target:
            lo = mid + 1
        else:
            hi = mid
    return lo


def merge_intervals(intervals):
    """Closed intervals; touching endpoints merge. Does not mutate input."""
    out = []
    for start, end in sorted(intervals):
        if start > end:
            raise ValueError('reversed interval')
        if out and start <= out[-1][1]:
            out[-1][1] = max(out[-1][1], end)
        else:
            out.append([start, end])
    return out


def top_k_frequent(nums, k):
    if k < 0:
        raise ValueError('k must be nonnegative')
    counts = Counter(nums)
    # Higher count wins; smaller value breaks ties deterministically.
    return nlargest(k, counts, key=lambda value: (counts[value], -value))


def islands(grid):
    """Rectangular list of lists of '0'/'1'; input remains unchanged."""
    if not grid:
        return 0
    cols = len(grid[0])
    if any(len(row) != cols or any(x not in ('0', '1') for x in row) for row in grid):
        raise ValueError('expected rectangular binary grid')
    seen = set()
    count = 0
    for r, row in enumerate(grid):
        for c, value in enumerate(row):
            if value != '1' or (r, c) in seen:
                continue
            count += 1
            seen.add((r, c))
            queue = deque([(r, c)])
            while queue:
                x, y = queue.popleft()
                for a, b in ((x-1, y), (x+1, y), (x, y-1), (x, y+1)):
                    if 0 <= a < len(grid) and 0 <= b < cols and grid[a][b] == '1' and (a, b) not in seen:
                        seen.add((a, b))  # Mark on enqueue, not dequeue.
                        queue.append((a, b))
    return count


def course_order(n, prerequisites):
    """Each pair is (course, prerequisite); [] means a cycle or no courses."""
    if n < 0:
        raise ValueError('negative course count')
    adjacency = [set() for _ in range(n)]
    degree = [0] * n
    for course, prerequisite in prerequisites:
        if not 0 <= course < n or not 0 <= prerequisite < n:
            raise ValueError('course outside graph')
        if course not in adjacency[prerequisite]:
            adjacency[prerequisite].add(course)
            degree[course] += 1
    queue = deque(i for i, d in enumerate(degree) if d == 0)
    order = []
    while queue:
        node = queue.popleft()
        order.append(node)
        for neighbor in adjacency[node]:
            degree[neighbor] -= 1
            if degree[neighbor] == 0:
                queue.append(neighbor)
    return order if len(order) == n else []


def shortest_paths(n, edges, source):
    """Directed nonnegative weighted graph; unreachable distances are infinity."""
    if not 0 <= source < n:
        raise ValueError('invalid source')
    graph = [[] for _ in range(n)]
    for u, v, weight in edges:
        if not 0 <= u < n or not 0 <= v < n or weight < 0:
            raise ValueError('invalid edge')
        graph[u].append((v, weight))
    distance = [float('inf')] * n
    distance[source] = 0
    heap = [(0, source)]
    while heap:
        cost, node = heappop(heap)
        if cost != distance[node]:
            continue
        for neighbor, weight in graph[node]:
            candidate = cost + weight
            if candidate < distance[neighbor]:
                distance[neighbor] = candidate
                heappush(heap, (candidate, neighbor))
    return distance


class LRU:
    """Capacity in entries; None denotes a miss. Not thread-safe."""
    def __init__(self, capacity):
        if capacity < 0:
            raise ValueError('negative capacity')
        self.capacity = capacity
        self.data = OrderedDict()

    def get(self, key):
        if key not in self.data:
            return None
        self.data.move_to_end(key)
        return self.data[key]

    def put(self, key, value):
        if self.capacity == 0:
            return
        self.data[key] = value
        self.data.move_to_end(key)
        if len(self.data) > self.capacity:
            self.data.popitem(last=False)


def min_coins(coins, amount):
    if amount < 0 or any(c <= 0 for c in coins):
        raise ValueError('nonnegative amount and positive coins required')
    dp = [0] + [amount + 1] * amount
    for total in range(1, amount + 1):
        for coin in coins:
            if coin <= total:
                dp[total] = min(dp[total], dp[total-coin] + 1)
    return -1 if dp[amount] > amount else dp[amount]


def daily_temperatures(temperatures):
    answer = [0] * len(temperatures)
    stack = []
    for i, temp in enumerate(temperatures):
        while stack and temperatures[stack[-1]] < temp:
            previous = stack.pop()
            answer[previous] = i - previous
        stack.append(i)
    return answer


def word_exists(board, word):
    """4-neighbor word search; do not reuse a cell or mutate the board."""
    if not word:
        return True
    if not board:
        return False
    cols = len(board[0])
    if any(len(row) != cols for row in board):
        raise ValueError('ragged board')
    def visit(r, c, index, used):
        if not (0 <= r < len(board) and 0 <= c < cols) or (r, c) in used or board[r][c] != word[index]:
            return False
        if index == len(word) - 1:
            return True
        used.add((r, c))
        found = any(visit(a, b, index+1, used) for a, b in ((r+1,c),(r-1,c),(r,c+1),(r,c-1)))
        used.remove((r, c))
        return found
    return any(visit(r, c, 0, set()) for r in range(len(board)) for c in range(cols))


class Trie:
    END = object()

    def __init__(self):
        self.root = {}

    def insert(self, word):
        node = self.root
        for char in word:
            node = node.setdefault(char, {})
        node[self.END] = True

    def contains(self, word):
        node = self.root
        for char in word:
            if char not in node:
                return False
            node = node[char]
        return self.END in node