20. Valid Parentheses
class Solution {
public:
bool isValid(string s) {
stack<char>sta;
for(int i=0;i<s.size();i++){
if(s[i]=='{'||s[i]=='['||s[i]=='('){
sta.push(s[i]);
continue;
}
if(sta.empty())return false;
if(sta.top()=='{'&&s[i]=='}'||
sta.top()=='('&&s[i]==')'||
sta.top()=='['&&s[i]==']'
)sta.pop();
else return false;
}
if(sta.empty())return true;
else return false;
}
};
1047. Remove All Adjacent Duplicates in a String
class Solution {
public:
string removeDuplicates(string s) {
string res;
for(char a:s){
if(res.empty()||res.back()!=a){
res+=a;
}else{
res.pop_back();
}
}
return res;
}
};
150. Reverse Polish expression evaluation
class Solution {
public:
int evalRPN(vector<string>& tokens) {
stack<long long>sta;
for(int i=0;i<tokens.size();i++){
if(tokens[i]=="+"||tokens[i]=="-"||tokens[i]=="*"||tokens[i]=="/"){
long long a=sta.top();
sta.pop();
long long b=sta.top();
sta.pop();
long long c=0;
if(tokens[i]=="+"){
c=a+b;
}else if(tokens[i]=="-"){
c=b-a;
}else if(tokens[i]=="*"){
c=a*b;
}else {
c=b/a;
}
sta.push(c);
}else{
long long count=stoll(tokens[i]);
sta.push(count);
}
}
return sta.top();
}
};