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Given a string s which represents an expression, evaluate this expression and return its value. The integer division should truncate toward zero. You may assume that the given expression is always valid. All intermediate results will be in the range of [-2^31, 2^31 - 1]. Note: You are not allowed to use any built-in function which evaluates strings as mathematical expressions.
Example 1
Input: s = 3+2*2
Output: 7
Explanation: Multiplication is done before addition, so 2 * 2 = 4 is evaluated first, then 3 + 4 = 7.
Example 2
Input: s = 3/2
Output: 1
Explanation: 3 / 2 = 1.5, and integer division truncates toward zero, so the result is 1.
Example 3
Input: s = 3+5 / 2
Output: 5
Explanation: Division binds tighter than addition: 5 / 2 = 2 (truncated), then 3 + 2 = 5.
Process * and / immediately by popping the stack. Push results of + and - for later summation.
Run checks all cases above. Submit evaluates all test cases.