You are given a string expression consisting of integers, '+', '-', '*', '/' operators, and spaces. Evaluate the expression and return the result as an integer. Integer division should truncate toward zero. The expression will always be valid. No parentheses are allowed.
3+2*27 3/2 1 3+5 / 2 514-3/2130-2147483647-2147483647["1 <= s.length <= 3 * 10^5", "s consists of integers and operators ('+', '-', '*', '/') separated by some number of spaces.", "s represents a valid expression.", "All the integers in the expression are non-negative integers in the range [0, 2^31 - 1].", "The result is guaranteed to fit in a 32-bit integer."]