## 1119. Pre- and Post-order Traversals (30)-PAT甲级真题（前序后序转中序）

Suppose that all the keys in a binary tree are distinct positive integers. A unique binary tree can be determined by a given pair of postorder and ino[……]

## 1122. Hamiltonian Cycle (25)-PAT甲级真题

The “Hamilton cycle problem” is to find a simple cycle that contains every vertex in a graph. Such a cycle is called a “Hamiltonian cycle”.

In this p[……]

## 1124. Raffle for Weibo Followers (20)-PAT甲级真题

John got a full mark on PAT. He was so happy that he decided to hold a raffle（抽奖） for his followers on Weibo — that is, he would select winners from e[……]

## PAT 1129. Recommendation System (25) 甲级

Recommendation system predicts the preference that a user would give to an item. Now you are asked to program a very simple recommendation system that[……]

## PAT 1128. N Queens Puzzle (20)-甲级

The “eight queens puzzle” is the problem of placing eight chess queens on an 8×8 chessboard so that no two queens threaten each other. Thus, a solutio[……]

## 1120. Friend Numbers (20)-PAT甲级真题

Two integers are called “friend numbers” if they share the same sum of their digits, and the sum is their “friend ID”. For example, 123 and 51 are fri[……]