Evaluate Reverse Polish Notation

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.

  Some examples:   ["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9   ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6

  • 题目大意:给定一个逆波兰表达式,求该表达式的值
  • 思路:由于逆波兰表达式本身不需要括号来限制哪个运算该先进行,因此可以直接利用栈来模拟计算:遇到操作数直接压栈,碰到操作符直接取栈顶的2个操作数进行计算(注意第一次取出来的是右操作数),然后再把计算结果压栈,如此循环下去。最后栈中剩下的唯一一个元素便是整个表达式的值
  • 实现:
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class Solution {
public:
    int evalRPN(vector<string> &tokens) {
         
        int result = 0;
        int i;
        stack<int> opd;         //存储操作数
        int size = tokens.size();
        for(i=0;i<size;i++)
        {
            if(tokens[i]=="*")
            {
                int rOpd = opd.top();   //右操作数
                opd.pop();
                int lOpd = opd.top();  //左操作数
                opd.pop();
                result = lOpd*rOpd;
                opd.push(result);
            }
            else if(tokens[i]=="/")
            {
                int rOpd = opd.top();
                opd.pop();
                int lOpd = opd.top();
                opd.pop();
                result = lOpd/rOpd;
                opd.push(result);
            }
            else if(tokens[i]=="+")
            {
                int rOpd = opd.top();
                opd.pop();
                int lOpd = opd.top();
                opd.pop();
                result = lOpd+rOpd;
                opd.push(result);
            }
            else if(tokens[i]=="-")
            {
                int rOpd = opd.top();
                opd.pop();
                int lOpd = opd.top();
                opd.pop();
                result = lOpd-rOpd;
                opd.push(result);
            }
            else
            {
                opd.push(atoi(tokens[i].c_str()));
            }
        }
        return opd.top();
    }
};

  

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