How should I practice Simplify a Binary Expression Tree?

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Simplify a Binary Expression Tree

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Problem statement

A valid binary expression tree is serialized in prefix order. Each token is +, -, *, a variable name, or a signed integer.

Simplify the tree bottom-up. Fold an operator when both children are integers. Also apply x + 0 = x, 0 + x = x, x - 0 = x, x * 1 = x, 1 * x = x, and x * 0 = 0. Return the simplified tree as one space-separated prefix expression.

Function

simplifyExpressionTree(preorder: String[]) → String

Examples

Example 1

preorder = ["+","*","x","1","0"]return = "x"

Multiplication by one and addition of zero both disappear.

Example 2

preorder = ["*","+","2","3","y"]return = "* 5 y"

The constant addition folds while the variable multiplication remains.

Constraints

1 <= preorder.length <= 10000.
The prefix sequence is a valid binary expression tree.
Every folded result fits in a signed 64-bit integer.