Inorder of a binary search tree is sorted
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inserted in this order : [50, 30, 70, 20, 40, 60, 80]
tree                   : 50(30(20,40),70(60,80))
preorder               : [50, 30, 20, 40, 70, 60, 80]
inorder                : [20, 30, 40, 50, 60, 70, 80]
postorder              : [20, 40, 30, 60, 80, 70, 50]