a.崎岖山路 b.凤凰岭 c.悬崖
          b-b   b           b-b-b b-b-b     b-b
         /   \ / \         /     \|   |\    |  \
        b   b-b   b   b   b       b c-b b-b-b-b-b
        |   |/   / \ / \ / \      |\   \   /|\ / \
        b   b-b-b-c b   b   b b   b b   b-b b b b-b
       /|\   \     /       / \|    \ \     / \ \ \|
  b-b-b b-b-b-b-b b-b-b b-b-b b-b-b-b-b   b   b b-b-b
 /     /     \ /  |  /    |/  |     |      \   \|  /
b-b   b       a   b b   b-b-b-b   b-b-b   b-b   b-b
 \ \ /|      /    |/ \ / /     \ / /   \ / /    |  \
  b-b b     a     b   b b       b b     b b b-b-b   b
 /    |    /      |  / / \     / /     / /  |  /   / \
b     b c a     a-a-a-a-a-a-a-a-a-a-a-a-a-a b-b-b b   b
 \    | | |     |     |  /       / \       /     \ \ /
b-b-b-b b a     a   a-a-a   a-a-a-a b-b-b b c b   b b
 /     / /|     |   |    \ /  |    \ /  |/ / /|\   \ \
b-b-b b b-a-a-a-a   a     a   a     b-b b b-b b b-b-b b
   \ \|  \      |   |\   /   /      |  \|\  |/ \|    /
    b-b-b-b     a-a-a a-a-a-a       b-b-b b-b   b-b-b
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                        a