Implement a basic calculator to evaluate a simple expression string.
The expression string contains only non-negative integers, +, -, *, / operators and empty spaces . The integer division should truncate toward zero.
You may assume that the given expression is always valid.
Some examples: Note: Do not use the eval built-in library function.
Credits: Special thanks to @ts for adding this problem and creating all test cases.
和Basic Calculator比較相似。都是很巧妙的解法。
runtime:24ms
class Solution { public: int calculate(string s) { int len=s.size(); if(s.empty()) return 0; stack<int> st; int num = 0; char sign = '+'; for(int i=0;i<len;i++){ if(isdigit(s[i])){ num = num*10+s[i]-'0'; } if(!isdigit(s[i]) &&' '!=s[i] || i==len-1){ if(sign=='-'){ st.push(-num); } if(sign=='+'){ st.push(num); } if(sign=='*'){ int tmp=st.top(); st.pop(); st.push(tmp*num); } if(sign=='/'){ int tmp=st.top(); st.pop(); st.push(tmp/num); } sign = s[i]; num = 0; } } int re = 0; while(!st.empty()){ re += st.top(); st.pop(); } return re; } };转载于:https://www.cnblogs.com/bhlsheji/p/5314037.html
相关资源：数据结构—成绩单生成器