这题只要理解RPN的原理就不难，用一个stack就行了。为虾米leetcode不提供atoi函数~~只好自己临时写个。注意负数的情况就可以了
1 class Solution { 2 public: 3 int stringtoint(string s) { 4 int ans = 0; 5 for (int i = isdigit(s[0])? 0 : 1; i < s.size(); i++) { 6 ans = ans*10 + s[i] - '0'; 7 } 8 return isdigit(s[0])? ans : -ans; 9 } 10 int evalRPN(vector<string> &tokens) { 11 int res = 0; 12 stack<int> S; 13 for (int i = 0; i < tokens.size(); i++) { 14 if (isdigit(tokens[i][0]) || tokens[i].size() > 1 && isdigit(tokens[i][1])) S.push(stringtoint(tokens[i])); 15 else { 16 int a = S.top(); 17 S.pop(); 18 int b = S.top(); 19 S.pop(); 20 if (tokens[i] == "+") S.push(b+a); 21 if (tokens[i] == "-") S.push(b-a); 22 if (tokens[i] == "*") S.push(b*a); 23 if (tokens[i] == "/") S.push(b/a); 24 } 25 } 26 return S.top(); 27 } 28 };
C#
1 public class Solution { 2 public int EvalRPN(string[] tokens) { 3 int ans = 0; 4 Stack<int> S = new Stack<int>(); 5 for (int i = 0; i < tokens.Length; i++) { 6 if (isdigit(tokens[i][0]) || tokens[i].Length > 1 && isdigit(tokens[i][1])) S.Push(stringtoint(tokens[i])); 7 else { 8 int a = S.Peek(); 9 S.Pop(); 10 int b = S.Peek(); 11 S.Pop(); 12 if (tokens[i] == "+") S.Push(b + a); 13 if (tokens[i] == "-") S.Push(b - a); 14 if (tokens[i] == "*") S.Push(b * a); 15 if (tokens[i] == "/") S.Push(b / a); 16 } 17 } 18 return S.Peek(); 19 } 20 public bool isdigit(char c) { 21 return c <= '9' && c >= '0'; 22 } 23 public int stringtoint(string s) { 24 int ans = 0; 25 for (int i = isdigit(s[0])? 0 : 1; i < s.Length; i++) { 26 ans = ans * 10 + s[i] - '0'; 27 } 28 return isdigit(s[0])? ans : -ans; 29 } 30 }