DIY: Basic Calculator II

Problem statement

In this challenge, you are given a string, st, which contains an expression. You’ll evaluate this expression and return its value. Keep the following statements in mind while solving the problem:

• The integer division should be rounded off to the nearest smallest integer.
• The given expression is always valid, giving the intermediate result in the range of $[$-$2^{31}$, $2^{31}$ - $1]$.
• You are not allowed to use any built-in function that evaluates the string as a mathematical expression.

Constraints:

• st will contain an expression made up of integers and operators (*,/, +, -) and separated by some number of spaces.
• The expression will contain non-negative integers in the range $[0$, $2^{31}$ - $1]$.
• $1 <=$ st.length $<= 3 \times 10^5$