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Remove K Digits in C++
Suppose we have a non-negative integer num that is represented as a string, we have to remove k digits from the number so that the new number is the smallest possible. So if the input is like “1432219” and k = 3, then the result will be “1219”.
To solve this, we will follow these steps −
Define a stack st, create an empty string ret
n := size of num
for i in range 0 to n – 1
while k is non zero and stack is not empty and top of stack > num[i]
delete from the stack and decrease k by 1
insert num[i] into st
while k is not 0, delete element from the stack
while the stack is not empty
ret := ret + top of stack, delete element from stack
now reverse the ret string
ans := an empty string, and i := 0
while i < size of ret and ret[i] is not ‘0’
increase i by 1
for i < size of ret
ans := ans + ret[i]
ret := ans
return “0” if size of ret is 0, otherwise, ret
Example (C++)
Let us see the following implementation to get a better understanding −
class Solution {
public:
string removeKdigits(string num, int k) {
stack st;
string ret = "";
int n = num.size();
for(int i = 0; i < n; i++){
while(k && !st.empty() && st.top() > num[i]){
st.pop();
k--;
}
st.push(num[i]);
}
while(k--)st.pop();
while(!st.empty()){
ret += st.top();
st.pop();
}
reverse(ret.begin(), ret.end());
string ans = "";
int i = 0;
while(i <ret.size() && ret[i] == '0')i++;
for(; i < ret.size(); i++)ans += ret[i];
ret = ans;
return ret.size() == 0? "0" : ret;
}
};Input
"1432219" 3
Output
"1219"
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