336. Left View of a Tree
Print the first node of each row, top to bottom. That is what you would see standing to the left of the tree and looking across it.
4
/ \
2 6 left view is 4 2 1
/ \ / \
1 3 5 7
Note that the left view is NOT the leftmost path. If the leftmost branch is short, a node from a different branch becomes the first thing visible on the rows below it, which is what the uneven test checks.
A level-order walk gives it directly: the first node reached at each new depth is the answer for that depth.
Constraints - `1 ≤ n ≤ 100000` - `-1000000000 ≤ value ≤ 1000000000` - The input always forms a valid tree rooted at node 1.
Input
The first line contains an integer n, the number of nodes.
The second line contains n values, where node i holds the ith value.
Each of the next n lines contains two integers, the left and right child of
node i, using 0 for no child. Node 1 is the root.
Output
Print the first value of each row, top to bottom, separated by single spaces.
Hints
Four rungs, in order. The last two open once you have submitted an attempt. A wrong one counts.