This function mainly creates an unordered_map , then calls buildTree ( )
struct Node * buldTreeWrap ( char in [ ] , char pre [ ] , int len ) { unordered_map < char , int > mp ; for ( int i = 0 ; i < len ; i ++ ) mp [ in [ i ] ] = i ; return buildTree ( in , pre , 0 , len - 1 , mp ) ; }