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A binary search tree is a binary tree data structure where each node is greater than all the nodes in its left sub-tree and smaller than all the nodes in its right sub-tree. Each child is another binary search tree.
The shape of a binary search tree depends on the order of insertions and it can be degenerate. More complex data structures like AVL-trees or red-black trees are self balancing binary search trees and ensure that the shape does not become degenerate leading to worst case run times.