889. construct binary tree according to preorder and postorder traversal

Blog explanation https://grandyang.com/leetcode/889/

The time complexity of a normal recursive approach is O(n^2), But at the end of the paper, we give the stack implementation O(n) Time complexity solution . But the answer is stuck to leetcode There will be a mistake .

Optimization of recursive approach , Can be introduced hashmap The nodes -> Index save , Appropriately speed up the search .