Lexicographically Smallest Subsequence- Important Java Coding Problem Solved & Explained

Lexicographically Smallest Subsequence- Important Java Coding Problem Solved & Explained With Example.

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Problem Description:
Return the lexicographically smallest subsequence of s of length k.

A subsequence is a string that can be derived from another string by deleting
some or no characters without changing the order of the remaining characters.

A string a is lexicographically smaller than a string b if in the first
position where a and b differ, string a has a letter that appears earlier in
the alphabet than the corresponding letter in b.

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[pranaymore]

thank u for the insides. it helped me alot

[pranaymore]

public String solve(String A) {
Stack<Character> stack = new Stack<>();
int N = A.length();

for(int i =0; i< N; i++){
if(stack.isEmpty()){
stack.add(A.charAt(i));
}
else{
while(!stack.isEmpty() && (A.charAt(i) < stack.peek()) && (stack.size()+N-i-1 >=2)){
stack.pop();
}

if(stack.isEmpty() || stack.size()<2){
stack.add(A.charAt(i));
}
}
}

StringBuilder sb = new StringBuilder();
while(!stack.isEmpty()){
sb.append(stack.peek());
stack.pop();
}
return sb.reverse().toString();


}