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:
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| Input: ["2", "1", "+", "3", "*"]
Output: 9
Explanation: ((2 + 1) * 3) = 9
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Example 2:
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| Input: ["4", "13", "5", "/", "+"]
Output: 6
Explanation: (4 + (13 / 5)) = 6
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Example 3:
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| 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
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Stack
感觉是很基础的题了, 难度设为medium有点不合适. 学数据结构的时候肯定学过这个题!!
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| class Solution {
public int evalRPN(String[] tokens) {
Deque<Integer> stk = new ArrayDeque<>();
for(String token : tokens)
{
if(!isSymbol(token))
{
stk.offerFirst(Integer.parseInt(token));
}
else
{
int b = stk.pollFirst();
int a = stk.pollFirst();
int res;
if("+".equals(token))
res = a + b;
else if("-".equals(token))
res = a - b;
else if("*".equals(token))
res = a * b;
else
res = a / b;
stk.offerFirst(res);
}
}
return stk.pollFirst();
}
private boolean isSymbol(String i)
{
return "+".equals(i) || "-".equals(i) || "*".equals(i) || "/".equals(i);
}
}
|