haskell最短路-自己实现fold代替递归

Paul的第二次作业,Use higher-order functions such as map and fold (or adaptations thereof for relevant structures) rather than explicit recursion.也就是用自己针对Pathtree进行fold定义,然后利用fold来替换递归.这个是这次作业的重点.占了三分之一的得分比例.