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DFS & BFS & Backtracking

DFS & BFS & Backtracking. N.S.Lin@ntnu.csie@Taiwan www.csie.ntnu.edu.tw/~u99256. CC – 非商業性 – 相同方式. Preface. DFS : Depth-First-Search BFS : Breadth-First-Search 常用 於圖形問題 後者常用來尋找最佳解. DFS. 沿著一條路走，遇到死路再回頭， 直到 得解或確定無 解 是一種 窮舉的方法 使用 Stack 利用 Recursion. D FS. (4 , 5). (4 , 4).

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DFS & BFS & Backtracking

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  1. DFS&BFS& Backtracking N.S.Lin@ntnu.csie@Taiwan www.csie.ntnu.edu.tw/~u99256 CC – 非商業性 – 相同方式

  2. Preface • DFS : Depth-First-Search • BFS : Breadth-First-Search • 常用於圖形問題 • 後者常用來尋找最佳解

  3. DFS • 沿著一條路走，遇到死路再回頭， 直到得解或確定無解 • 是一種窮舉的方法 • 使用 Stack • 利用 Recursion

  4. DFS (4 , 5) (4 , 4) (1 , 4) (4 , 3) (2 , 4) (4 , 2) (2 , 2) (4 , 1) (2 , 2) (3 , 1) (2 , 1) (1 , 1)

  5. DFS function DFS( node ) if( got-end ){ print:"I got it!" } if can-go( node.up)｛　go( node.up)　｝ if can-go( node.right)｛　go( node.right)　｝ if can-go( node.left)｛　 go( node.right ) 　｝ if can-go( node.down)｛ go( node.down)　｝ end

  6. BFS • 同時嘗試每條路徑 • 是一種擴散的概念 • 使用 Queue • 常用於尋找最佳解

  7. BFS (5 , 1) (4 , 2) (2 , 4) (4 , 1) (2 , 3) (5 , 4) (3 , 1) (4 , 4) (2 , 2) (4 , 3) (2 , 1) (1 , 1) (1 , 4)

  8. Backtracking • 在窮舉的時候多做一些檢查判斷， 避免浪費時間在探尋不可能的路徑上

  9. 8-Queen Problem

  10. 8-Queen Problem • 方法一 -暴力硬做, 每種擺法都試一次 • 8 * 8 棋盤上：64 * 63 * …. * 57(次) • 方法二 – 以 Backtracking 加快速度 • 每一列只擺一隻皇后 • 8* 8 棋盤上：8 * 7 * 6 * 5 * 4 * 3 * 2 * 1(次) • 比較次數減為 1/4426165368 倍

  11. Use C++ STL to easy ur work • Stack : #include <stack> … std::stack<(DATA_TYPE)> stack_1 stack_1.push() stack_1.top() stack_1.pop() stack_1.empty() stack_1.size() C++ Reference : http://www.cplusplus.com/reference/stl/stack/

  12. Use C++ STL to easy ur work • Queue : #include <queue> … std::queue<(DATA_TYPE)> queue_1 queue_1.push() queue_1.front() queue_1.back() queue_1.pop() queue_1.empty() queue_1.size() C++ Reference : http://www.cplusplus.com/reference/stl/queue/

  13. Use C++ STL to easy ur work 執行結果： C++ Reference : http://www.cplusplus.com/reference/stl/queue/

  14. Test Yourself • 10336 - Rank the Languages • 439 - Knight Moves • 750 - 8 Queens Chess Problem • 11352 - Crazy King

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