In the process of traversing a binary tree, we usually traverse the left subtree first, and then the right subtree. Under the principle of left first and then right, the traversal of binary tree is divided into three types: root node + left subtree + right subtree. When traversing the left subtree and the right subtree, you still access the root node first, then traverse the left subtree, and finally traverse the right subtree. Middle order traversal: left subtree + root node + right subtree. When traversing the left and right subtrees, we still traverse the left subtree, then the root node, and then the right subtree. Post order traversal: left subtree + right subtree + root node. When traversing the left and right subtrees, the left subtree is traversed first, the right subtree is traversed, and then the root node is accessed.