Basic Calculator

该题的思路很明确就是将中缀表达式转换为后缀表达式，然后通过后缀表达式来求值。

class Solution {
public:

int calculate(string s) {
vector<string> postorder;
stack<char> ccache;
stack<int> icache;
string tmp;
int len = s.length();
if(s.length() < 1)
return 0;
//构造后缀表达式
for(int i = 0; i < len; ){
if(s[i] == ' '){
i++;
continue;
}
if(s[i] == '+' || s[i] == '-' || s[i] == '(' || s[i] == ')'){
if(s[i] == '(' || ccache.empty()){
ccache.push(s[i]);
i++;
continue;
}
if(s[i] == ')'){
while(ccache.top() != '('){
tmp = "";
tmp += ccache.top();
postorder.push_back(tmp);
ccache.pop();
}
ccache.pop();
i++;
continue;
}
if(s[i] == '+' || s[i] == '-'){
while(!ccache.empty() && ccache.top() != '(' ){
tmp = "";
tmp += ccache.top();
postorder.push_back(tmp);
ccache.pop();
}
ccache.push(s[i]);
i++;
continue;
}
}else{
int j = i;
while(j < len && s[j] >= '0' && s[j] <= '9')
j++;
tmp = "";
tmp = s.substr(i, j - i);
postorder.push_back(tmp);
i = j;
}
}
//将栈中剩余的元素全部添加到后缀表达式字串中
while(!ccache.empty()){
tmp = "";
tmp += ccache.top();
ccache.pop();
postorder.push_back(tmp);
}

//通过后缀表达式求值
int fir, sec;
for(int i = 0; i < postorder.size(); i++){
if(postorder[i] == "+" || postorder[i] == "-"){
sec = icache.top();
icache.pop();
fir = icache.top();
icache.pop();
if(postorder[i] == "+")
icache.push(fir + sec);
if(postorder[i] == "-")
icache.push(fir - sec);
}else{
icache.push(atoi(postorder[i].c_str()));
}
}
return icache.top();
}
}