Minimum Deletions to Make String Balanced - c++

Question

Minimum Deletions to Make String Balanced - LeetCode
1653. Minimum Deletions to Make String Balanced
Medium
You are given a string s consisting only of characters 'a' and 'b'​​​​.
You can delete any number of characters in s to make s balanced. s is balanced if there is no pair of indices (i,j) such that i < j and s[i] = 'b' and s[j]= 'a'.
Return the minimum number of deletions needed to make s balanced.
Example 1:
Input: s = "aababbab"
Output: 2
Explanation: You can either:
Delete the characters at 0-indexed positions 2 and 6 ("aababbab" -> "aaabbb"), or
Delete the characters at 0-indexed positions 3 and 6 ("aababbab" -> "aabbbb").
Example 2:
Input: s = "bbaaaaabb"
Output: 2
Explanation: The only solution is to delete the first two characters.
Constraints:
1 <= s.length <= 105
s[i] is 'a' or 'b'​​.
class Solution {
public:
int minimumDeletions(string s) {
int cnt=0;
stack<int> st;
st.push(s[0]);
for(int i=1;i<s.length();i++){
if(s[i]=='b'){
st.push(s[i]);
}
else{
if(st.empty()==true){
st.push('a');
}
else if(st.top()=='a'){
st.push('a');
}
else{
while(st.empty()==false and st.top()=='b'){
cnt++;
st.pop();
}
st.push('a');
}
}
}
return cnt;
}
};
for both the exapmles the answer should be 2,2
but my code it giving 3,2

Answer 1

I recommend to learn some test framework. IMO two best are:
gtest - industry standard - old but quite powerful
catch2 - nice and more user friendly, quickly gains popularity.
Write test code which will validate your solution then use it with a debugger to find issue in your code.
Here is an example with catch2:
https://godbolt.org/z/xevvPrEeK
TEST_CASE("Sum") {
auto [result, s] = GENERATE(table<int, std::string>({
{ 0, ""},
{ 0, "a"},
{ 0, "b"},
{ 0, "ab"},
{ 1, "ba"},
{ 1, "bba"},
{ 1, "baa"},
{ 1, "aba"},
{ 2, "aababbab" },
{ 2, "bbaaaaabb" },
}));
INFO("s = " << s);
REQUIRE(result == Solution{}.minimumDeletions(s));
}
Note this test finds simplest case where your code fails.
On problem: use of the stack is obsolete. Just try find place where you have to delete all bs before it and all as after it.