03. Build an Array With Stack Operations

The problem can be found at the following link: Question Link

My Approach

1. Initialize an empty vector of strings named operations to store the sequence of operations.

2. Initialize an integer variable targetIndex to 0, which will keep track of the current index in the target array.

3. Inside the for loop:

   • Check if targetIndex is equal to the size of the target array. If it is, there are no more elements to match in the target array, so exit the loop by using a break statement.

4. Check if the current element in the target array at index targetIndex is equal to i. If it is, it means the element at index targetIndex in the target array matches the current value i in the range 1 to n. In this case:

   • Append "Push" to the operations vector to represent pushing the current value i onto the stack.

   • Increment targetIndex to move to the next element in the target array.

5. If the current element in the target array at index targetIndex is not equal to i, it means that the element at index targetIndex in the target array does not match the current value i in the range 1 to n. In this case:

   • Append "Push" to the operations vector to represent pushing the current value i onto the stack.

   • Append "Pop" to the operations vector to represent popping an element from the stack.

6. Continue the loop until all elements in the target array have been matched or until the loop reaches the end of the range from 1 to n.

7. After the loop, return the operations vector, which contains the sequence of operations required to transform an array into the target array using a stack.

Time and Auxiliary Space Complexity

• Time Complexity: O(n)

• Auxiliary Space Complexity: O(n)

Code (C++)

class Solution {
public:
    vector<string> buildArray(vector<int>& target, int n) {
        vector<string> operations;
        
        int targetIndex = 0;
        
        for (int i = 1; i <= n; i++) {
            if (targetIndex == target.size()) {
                break;  // No more elements to match in the target array
            }
            
            if (target[targetIndex] == i) {
                operations.push_back("Push");
                targetIndex++;
            } else {
                operations.push_back("Push");
                operations.push_back("Pop");
            }
        }
        
        return operations;
    }

};

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