359. Size of a Subtree
Print how many nodes lie in the subtree rooted at the node holding value x,
counting that node itself. Print 0 if no node holds that value.
Two steps: find the node, then count everything beneath it. Both are traversals you have already written, and the only new idea is that the second one starts somewhere other than the root.
Counting the starting node itself is the part to get right. A subtree of a leaf has size 1, not 0.
Constraints - `1 ≤ n ≤ 100000` - `-1000000000 ≤ value, x ≤ 1000000000` - All values are distinct. - The input always forms a valid tree rooted at node 1.
Input
The first line contains two integers n and x.
The second line contains n values, where node i holds the ith value.
Each of the next n lines contains the left and right child of node i,
using 0 for no child. Node 1 is the root.
Output
Print one integer, the number of nodes in that subtree, or 0 if x is absent.
Hints
Four rungs, in order. The last two open once you have submitted an attempt. A wrong one counts.