150. Evaluate Reverse Polish Notation
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: ["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
题意：逆波兰式，又一道经典的 stack 题
代码
class Solution {
public int evalRPN(String[] tokens) {
Stack<Integer> stack = new Stack<>();
for(int i = 0; i < tokens.length; i++) {
if(!tokens[i].equals("+") && !tokens[i].equals("-") && !tokens[i].equals("*") && !tokens[i].equals("/")) {
stack.push(Integer.valueOf(tokens[i]));
continue;
} else {
int val1 = stack.pop();
int val2 = stack.pop();
if(tokens[i].equals("+")) stack.push(val1 + val2);
if(tokens[i].equals("*")) stack.push(val1 * val2);
if(tokens[i].equals("/")) stack.push(val2/val1);
if(tokens[i].equals("-")) stack.push(val2 - val1);
}
}
return stack.pop();
}
}