Print the Smallest String through a Last-In-First-Out Robot Buffer
Company: Bloomberg
Role: Software Engineer
Category: Coding & Algorithms
Difficulty: medium
Interview Round: Other
### Problem
Start with input `s`, an empty robot buffer t, and empty paper. You may move the first remaining input character to the end of t, or remove the last character of t and append it to the paper. Continue until both s and t are empty. Return the lexicographically smallest possible paper string.
```cpp
class Solution {
public:
string robotWithString(string s);
};
```
### Examples
**Example 1:**
```text
Input: s = "cba"
Output: "abc"
Explanation: Push all three characters, then pop them in reverse order.
```
**Example 2:**
```text
Input: s = "abac"
Output: "aabc"
Explanation: Print the first a immediately, then push b and a; print a then b before reading and printing c.
```
### Constraints
- `1 <= s.length <= 100000`.
- s contains lowercase English letters.
### Clarifying Questions
- Which input character may be taken next? Only the first remaining one.
- Which buffer character may be printed? Only the last one.
- Can any character be discarded? No; both input and buffer must end empty.
- Is any valid output acceptable? No; return the lexicographically smallest.
```hint Check the boundary
Check the order of characters after putting several into the buffer before printing any.
```
Overview: Find the lexicographically smallest string printable by moving input characters into a buffer and printing only its last character.