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A binary expression tree is a binary tree in which the leaves of the tree are operands (numbers), and the internal nodes are operators (+, -, *, /). For example, if we represent the expression "(3 + 2) * (4 - 1)" as a binary expression tree, it will look like: ``` * / \ + - / \ / \ 3 2 4 1 ```

We will write a recursive function `evaluate_expression_tree(node)` to evaluate the given binary expression tree. This function takes a node as input and returns the result of the expression represented by the subtree rooted at this node. The base case of the recursion will be when we encounter a leaf node (which is an operand), at which point we can return its value. ```python def evaluate_expression_tree(node): if node is None: return 0 # If the node is a leaf (number), return its value if node.left is None and node.right is None: return int(node.value) ```

Now, we need to implement the logic to process the internal nodes (operators) in a post-order manner, i.e., process the left subtree, then the right subtree, and finally the current node. For this, we will call the `evaluate_expression_tree` function recursively for the left and right children, and then perform the operation specified by the current node. ```python # Recursively evaluate the left and right subtree left_result = evaluate_expression_tree(node.left) right_result = evaluate_expression_tree(node.right) # Perform the operation specified by the current node if node.value == '+': return left_result + right_result elif node.value == '-': return left_result - right_result elif node.value == '*': return left_result * right_result elif node.value == '/': return left_result / right_result ``` With these steps, we have defined the algorithm to evaluate a binary expression tree. The final implementation of the `evaluate_expression_tree` function will look like this: ```python def evaluate_expression_tree(node): if node is None: return 0 # If the node is a leaf (number), return its value if node.left is None and node.right is None: return int(node.value) # Recursively evaluate the left and right subtree left_result = evaluate_expression_tree(node.left) right_result = evaluate_expression_tree(node.right) # Perform the operation specified by the current node if node.value == '+': return left_result + right_result elif node.value == '-': return left_result - right_result elif node.value == '*': return left_result * right_result elif node.value == '/': return left_result / right_result ```