Unexpected answer for a simple stack problem in C++ - c++

Question

class Solution {
public:
bool isValid(string s) {
map<char , char> m;
m[')'] = '(';
m['}'] = '{';
m[']'] = '[';
stack<char> st;
if(s[0] != '(' || s[0] != '{' || s[0] != '[')
return "false";
for(int i = 0; i<s.length(); i++)
{
if(s[i] == '(' || s[i]== '{' || s[i]== '[')
{
st.push(s[i]);
}
else if(st.top() == m[s[i]])
{
st.pop();
}
else if(st.top() != m[s[i]])
{
return "false";
}
}
if(st.empty())
return "true";
else
return "false";
}
};
The code fails for a basic example such as "(]". I don't understand how this is possible.
( first goes in the stack
( is not the map for ]
So it should return "false". But it returns true.

Answer 1

Regarding your expression:
(s[0] != '(' || s[0] != '{' || s[0] != '[')
For a start, unless s[0] is some weird quantum variable that can be three things at once, that expression will never be false. Think about some possibilities:
Character != '(' != '{' != '[' || all
--------- ------ ------ ------ ------
( false true true true
{ true false true true
[ true true false true
x true true true true
As you can see, every character will have that expression evaluating as true. You probably should be using && rather than ||.
In addition, you return a C-style string from your function which, being a non-zero pointer, will translate to true in a boolean context, as evidenced by:
#include <iostream>
bool returnT() { return "true"; }
bool returnF() { return "false"; }
int main() {
std::cout << returnT() << '\n';
std::cout << returnF() << '\n';
}
which outputs:
1
1
So I would be more inclined to start with something like this:
class Solution {
public:
bool isValid(string s) {
stack<char> st;
map<char, char> m;
m[')'] = '('; m['}'] = '{'; m[']'] = '[';
if (s[0] != '(' && s[0] != '{' && s[0] != '[') {
return false;
}
for (int i = 0; i < s.length(); ++i) {
if (s[i] == '(' || s[i]== '{' || s[i]== '[') {
st.push(s[i]);
} else if (st.top() == m[s[i]]) {
st.pop();
} else if (st.top() != m[s[i]]) {
return false;
}
}
return st.empty();
}
};
But do watch out for one thing. If your valid input characters are limited to the six brackets, you should be okay.
But if you want to allow for other characters that don't affect the stack, such as with:
[14^{6+(2x3)}]
then your code will not work because the first 1 character will be considered a mismatch. To handle that, you'll need to modify it to take them into account.
For example:
for (int i = 0; i < s.length(); ++i) {
if (s[i] == '(' || s[i]== '{' || s[i]== '[') {
// Handle open brackets: store.
st.push(s[i]);
} else if (s[i] == ')' || s[i]== '}' || s[i]== ']') {
// Handle close brackets: check.
if (st.top() == m[s[i]]) {
st.pop();
} else {
return false;
}
}
// Anything else is just a "noise" character, ignore.
}

Answer 2

Apparently, a ')' must be close to '(' in ascii code.
class Solution {
public:
bool isValid(string s) {
stack<char> st; st.push('\0');
for (auto c : s) {
if (isalpha(c)) continue;
else if (c - st.top() <= 3 && c - st.top() > 0)
st.pop();
else
st.push(c);
}
return st.size() == 1;
}
};
int main() {
Solution solu;
cout << solu.isValid("[](){}") << endl;
cout << solu.isValid(")(") << endl;
}