150. Evaluate Reverse Polish Notation
https://leetcode.com/problems/evaluate-reverse-polish-notation/
Intuition
Stack<Integer>
push to stack if numbers
pay attention to negative numbers identification
string starts with - and length > 1
when operator is encountered pop twice
order is important for division
second = stack.pop
first = stack.pop
---
Time - O(n)
Space - O(n)
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Evaluate the value of an arithmetic expression in Reverse Polish Notation.
Valid operators are
+, -, *, /. Each operand may be an integer or another expression.
Note:
- Division between two integers should truncate toward zero.
- The given RPN expression is always valid. That means the expression would always evaluate to a result and there won't be any divide by zero operation.
Example 1:
Input: ["2", "1", "+", "3", "*"] Output: 9 Explanation: ((2 + 1) * 3) = 9
Example 2:
Input: ["4", "13", "5", "/", "+"] Output: 6 Explanation: (4 + (13 / 5)) = 6
Example 3:
Input: ["10", "6", "9", "3", "+", "-11", "*", "/", "*", "17", "+", "5", "+"] Output: 22 Explanation: ((10 * (6 / ((9 + 3) * -11))) + 17) + 5 = ((10 * (6 / (12 * -11))) + 17) + 5 = ((10 * (6 / -132)) + 17) + 5 = ((10 * 0) + 17) + 5 = (0 + 17) + 5 = 17 + 5 = 22---
Intuition
Stack<Integer>
push to stack if numbers
pay attention to negative numbers identification
string starts with - and length > 1
when operator is encountered pop twice
order is important for division
second = stack.pop
first = stack.pop
---
Time - O(n)
Space - O(n)
---
