A vertex cover of an undirected graph is a subset of its vertices such that for every edge (u, v) of the graph, either ‘u’ or ‘v’ is in vertex cover.
Given a binary tree with N nodes, find and return the minimum of nodes in its vertex cover.
The first and the only argument has a pointer to the root node of the binary tree.
Return an integer, representing the answer as described in the question.
1 <= N <= 1e5
Input 1: 1 / \ 15 2 Output 1: 1 Explanation 1:  is enough to be the vertex cover of the tree. Input 2: 1 Output 2: 0 Explanation 2:  is enough to be the vertex cover of the tree. Input 3: 9 / \ 6 17 / \ 23 7 Output 3: 2 Explanation 3: [9, 6] is enough to be the vertex cover of the tree.
NOTE: You only need to implement the given function. Do not read input, instead use the arguments to the function. Do not print the output, instead return values as specified. Still have a doubt? Checkout Sample Codes for more details.