/**
* @param expression: a vector of strings;
* @return: an integer
*/
int evaluateExpression(vector<string> &expression) {
// write your code here
if(expression.size() == 0){
return 0;
}
// 1. converted to reversed polish expression
vector<string> rpe = convertToRPN(expression);
// 2. caculate the reversed polish expression
return calculateRPN(rpe);
}
int evalRPN(vector<string> &tokens) {
stack<int> s;
for (vector<string>::const_iterator it = tokens.begin(); it != tokens.end(); ++it) {
//如果是数字
if (isNumber(*it))
s.push(stoi(*it)); //将数字压入栈中
//如果是运算符
else if (*it == "+" || *it == "-" || *it == "*" || *it == "/") {
if (!s.empty()) { //如果栈中有数字
int opnd2 = s.top(); //获得表达式中的右边的操作数
s.pop(); //弹出
int opnd1 = s.top(); //获得左边的操作数
s.pop(); //弹出
s.push(calculate(opnd1, it->c_str(), opnd2)); //计算表达式，将结果再压入栈中
}
}
//如果是表达式
else {
vector<string> anotherTokens;
s.push(evalRPN(anotherTokens = convertToRPN(*it))); //如果是表达式那么转化成逆波兰式，在调用evalRPN，所得结果压入栈中
}
}
return s.top(); //返回最终结果
}
/**
* @param expression: A string array
* @return: The root of expression tree
*/
ExpressionTreeNode* build(vector<string> &expression) {
// write your code here
vector<string> rpn = convertToRPN(expression);
stack<ExpressionTreeNode*> stk;
for(auto e : rpn){
if(e == "+" || e == "-" || e == "*" || e == "/"){
ExpressionTreeNode* r = stk.top();
stk.pop();
ExpressionTreeNode* l = stk.top();
stk.pop();
ExpressionTreeNode* node = new ExpressionTreeNode(e);
node->left = l;
node->right = r;
stk.push(node);
}else{
ExpressionTreeNode* node = new ExpressionTreeNode(e);
stk.push(node);
}
}
if(stk.size())
return stk.top();
else
return NULL;
}