269. Alien Dictionary

解題說明

C++ 解法

複雜度分析

虛擬碼

1. Collect all unique characters, init in-degree to 0
2. For each adjacent word pair (w1, w2):
   a. Find first differing position j
   b. Add edge w1[j] -> w2[j], increment in-degree of w2[j]
   c. If w1 is longer prefix of w2: return "" (invalid)
3. Kahn's BFS: enqueue chars with in-degree 0, process, decrement neighbors
4. If all chars processed, return order; else return "" (cycle)

其他解法

延伸思考