Left View Of Binary Tree

Moderate

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Average time to solve is 30m

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Problem statement

You have been given a Binary Tree of 'n' nodes, where the nodes have integer values

Print the left view of the binary tree.

Example :

If the input tree is as depicted in the picture:

The Left View of the tree will be:  2 35 2

Detailed explanation ( Input/output format, Notes, Images )

Input format :

Elements in the level order form. The input consists of values of nodes separated by a single space in a single line. In case a node is null, we take -1 in its place.

For example, the input for the tree depicted in the below image would be :

1
2 3
4 -1 5 6
-1 7 -1 -1 -1 -1
-1 -1

Explanation :

Level 1 :
The root node of the tree is 1

Level 2 :
Left child of 1 = 2
Right child of 1 = 3

Level 3 :
Left child of 2 = 4
Right child of 2 = null (-1)
Left child of 3 = 5
Right child of 3 = 6

Level 4 :
Left child of 4 = null (-1)
Right child of 4 = 7
Left child of 5 = null (-1)
Right child of 5 = null (-1)
Left child of 6 = null (-1)
Right child of 6 = null (-1)

Level 5 :
Left child of 7 = null (-1)
Right child of 7 = null (-1)

The first not-null node (of the previous level) is treated as the parent of the first two nodes of the current level. The second not-null node (of the previous level) is treated as the parent node for the next two nodes of the current level, and so on.

The input ends when all nodes at the last level are null (-1).

Note :

The above format was just to provide clarity on how the input is formed for a given tree. 

The sequence will be put together in a single line separated by a single space. Hence, for the above-depicted tree, the input will be given as:

1 2 3 4 -1 5 6 -1 7 -1 -1 -1 -1 -1 -1

Output Format :

The only line of output will print the data in the Left View of the Tree separated by a single space.

Note:

You don’t need to print anything; It has already been taken care of.

Sample Input 1 :

2 35 10 2 3 5 2 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1

Sample Output 1 :

2 35 2

Explanation of Sample Input 1 :

The test case is explained in the problem statement.

Sample Input 2 :

1 2 3 4 5 -1 7 -1 -1 -1 -1 -1 -1

Sample Output 2 :

1 2 4

Explanation of Sample Input 2 :

The Tree looks as follows:
            1
          /   \
         2     3
       /   \     \
      4     5     7

Expected time complexity:

The expected time complexity is O(n).

Constraints :

0 <= 'n' <= 10^5
1 <= 'data' <= 10^5

Where ‘n’ is the total number of nodes in the binary tree, and 'data' is the value of the binary tree node.

Time limit: 1sec

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