Evaluate the value of an arithmetic expression in Reverse Polish Notation.
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题目链接:
https://leetcode.com/problems/evaluate-reverse-polish-notation/description/
描述
Evaluate the value of an arithmetic expression in Reverse Polish Notation.
Valid operators are +, -, *, /. Each operand may be an integer or another expression.
输入
[“2”, “1”, “+”, “3”, ““] -> ((2 + 1) 3) -> 9
[“4”, “13”, “5”, “/”, “+”] -> (4 + (13 / 5)) -> 6
输出
样例输入
样例输出
算法思想:
方法一:用栈来处理,从左向右依次扫描,如果遇到数字压栈,如果遇到加减乘除,从栈中弹出2个数字,进行运算,结果压入栈,直至扫描完成,栈里的最后结过就是运算结果
方法二:利用递归原理,
方法三:从左向右扫描,并压栈,用2个数保存每次扫描的最后2个数,当遇到加减乘除的时候,用这两个数运算后压栈,最后的就是最后结果
源代码
//方法一class Solution {public: int evalRPN(vector<string> &tokens) { stack<string> str; int a, b; // int result; for (int i = 0; i < tokens.size(); i++) { if (tokens[i].size() == 1 && ("0">tokens[i] || "9"<tokens[i])) //if is +-*/ { char ch = tokens[i].c_str()[0]; a = stoi(str.top()); str.pop(); b = stoi(str.top()); str.pop(); switch (ch) { case '+': a = a + b; break; case '-': a = b - a; break; case '*': a = a * b; break; case '/': a = b / a; break; default: break; } stringstream ss; ss << a; str.push(ss.str()); } else //when it is number { str.push((tokens[i])); } } return stoi(str.top()); }};//方法二class Solution{ public: int evalRPN(vector<string> &tokens) { string s = tokens.back(); tokens.pop_back(); if ( s== "*" || s=="/" || s=="+" || s == "-" ){ int r2 = evalRPN(tokens); int r1 = evalRPN(tokens); if ( s=="*") return r1*r2; if ( s=="/") return r1/r2; if ( s=="+") return r1+r2; if ( s=="-") return r1-r2; } else return atoi(s.c_str()); }};//方法三class Solution {public: int evalRPN(vector<string> &tokens) { stack<int> st; int s1,s2; s1=s2=0; int res=0; for(vector<string>::iterator iter=tokens.begin();iter!=tokens.end();iter++) { if (*iter == "+") { s1=st.top(); st.pop(); s2=st.top(); st.pop(); res=s1+s2; st.push(res); } else if (*iter == "-") { s1=st.top(); st.pop(); s2=st.top(); st.pop(); res=s2-s1; st.push(res); } else if (*iter == "*") { s1=st.top(); st.pop(); s2=st.top(); st.pop(); res=s1*s2; st.push(res); } else if (*iter== "/") { s1=st.top(); st.pop(); s2=st.top(); st.pop(); res=s2/s1; st.push(res); } else { st.push(atoi((*iter).c_str())); } } return st.top(); }};
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