Evaluate Reverse Polish Notation (M)
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
題意
計算逆波蘭表達式(即字尾表達式)的值。
思路
遇到數字就壓棧;遇到符号則從棧中彈出兩個數字進行運算(注意後出棧的在前,先出棧的在後),将得到的結果再壓入棧中;最後棧中隻剩一個數,即所求結果。
代碼實作
class Solution {
public int evalRPN(String[] tokens) {
Deque<Integer> stack = new ArrayDeque<>();
for (String token : tokens) {
if (token.equals("+")) {
int y = stack.pop(), x = stack.pop();
stack.push(x + y);
} else if (token.equals("-")) {
int y = stack.pop(), x = stack.pop();
stack.push(x - y);
} else if (token.equals("*")) {
int y = stack.pop(), x = stack.pop();
stack.push(x * y);
} else if (token.equals("/")) {
int y = stack.pop(), x = stack.pop();
stack.push(x / y);
} else {
stack.push(Integer.parseInt(token));
}
}
return stack.pop();
}
}