Evaluate Reverse Polish Notation--LeetCode

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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

思路：使用栈这个数据结构，只不过需要字符串和整数整数和字符串之间的转换

#include <iostream>#include <vector>#include <string>#include <stack>using namespace std;int strtoi(string& s){    if(s.length() == 0)      return 0;    int i=0,flag=1,result=0;    if(s[0]=='-')    {       flag =-1;       i++;       }    if(s[0]=='+')    {       i++;        }    for(;i<s.length();i++)    {        result *= 10;        result += (s[i]-'0');                     }    return result*flag;}string itostr(int num){    int flag =1,count = num;    if(num <0)    {        flag = 0;        count = -count;       }         string result;    while(count)    {      result += (count%10 +'0');      count = count/10;    }    if(flag == 0)     result += '-';    reverse(result.begin(),result.end());    return result;   }int evalRPN(vector<string> &tokens) {   stack<string> sk;   int result =0,i,temp;   string first,second;   for(i=0;i<tokens.size();i++)   {       if(tokens[i]=="+"||tokens[i]=="-"||tokens[i]=="*"||tokens[i]=="/")       {          first = sk.top();          sk.pop();          second =sk.top();          sk.pop();          if(tokens[i] == "+")             temp = strtoi(first)+strtoi(second);          else if(tokens[i]=="-")             temp = strtoi(first)-strtoi(second);                    else if(tokens[i]=="*")             temp = strtoi(first)*strtoi(second);          else             temp = strtoi(second)/strtoi(first);          first = itostr(temp);          sk.push(first);       }        else        sk.push(tokens[i]);                          }   first = sk.top();   result = strtoi(first);   return result;     }int main(){    string array[]={"4", "13", "5", "/", "+"};    vector<string> tokens(array,array+sizeof(array)/sizeof(array[0]));    cout<<evalRPN(tokens)<<endl;    system("pause");    return 0;        }

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