需要用回溯算法的题目：n皇后问题、素数方格问题、构造字符串、迷宫问题（包含题目及详细解答）。

回溯法的优化：递归前对尚待搜索的信息进行预处理、记忆化搜索。-Backtracking algorithms needed topics: n queens problem, prime squares problem, construct a string, the maze problem (including the title and a detailed answer).

Optimization of backtracking: Before recursive search for information on pending pretreatment, memory search.