1441. Build an Array With Stack Operations

T: O(n)
S: O(1)

```java
class Solution {
    public List<String> buildArray(int[] target, int n) {
        int tIndex = 0;
        int i = 1;
        List<String> result = new ArrayList<>();
        while (tIndex < target.length && i <= n) {
            if (target[tIndex] == i) {
                result.add("Push");
                tIndex++;
            } else {
                result.add("Push");
                result.add("Pop");
            }
            i++;
        } 
        return result;
    }
}


/***
T: O(n)
S: O(1)

in short, when target value not in [1...n] -> add result with "Push" and "Pop"
others all add "Push"

3
1

   pushpop pushpop
1 2 3 4.5 6 7 
1 2     5   7


 */
```