Kth Ancestor of a Tree Node

You are given a tree with n nodes numbered from 0 to n-1 in the form of a parent array where parent[i] is the parent of node i. The root of the tree is node 0.

Implement the function getKthAncestor(int node, int k) to return the k-th ancestor of the given node. If there is no such ancestor, return -1.

The k-th ancestor of a tree node is the k-th node in the path from that node to the root.

Example:

Input:
["TreeAncestor","getKthAncestor","getKthAncestor","getKthAncestor"]
[[7,[-1,0,0,1,1,2,2]],[3,1],[5,2],[6,3]]

Output:
[null,1,0,-1]

Explanation:
TreeAncestor treeAncestor = new TreeAncestor(7, [-1, 0, 0, 1, 1, 2, 2]);

treeAncestor.getKthAncestor(3, 1);  // returns 1 which is the parent of 3
treeAncestor.getKthAncestor(5, 2);  // returns 0 which is the grandparent of 5
treeAncestor.getKthAncestor(6, 3);  // returns -1 because there is no such ancestor

Constraints:

1 <= k <= n <= 5*10^4
parent[0] == -1 indicating that 0 is the root node.
0 <= parent[i] < n for all 0 < i < n
0 <= node < n
There will be at most 5*10^4 queries.

Solution:

class TreeAncestor {
    /*
    dp state: 
        dp[i][j] = j-th node's 2^(i)th ancestor in the path 
    dp init: 
        dp[i][j] = dp[0][j] (first parent (2^0) of each node is given)
    dp trans:
        dp[i][j] = dp[i-1][ dp[i-1][j] ] 
            meaning: A(j, 2^i) = A( A(j, 2^i-1), 2^i-1)
                To find the (2^i)-th ancestor of j, recursively find j-th node's 2^(i-1)th ancestor's 2^(i-1)th ancestor. (2^(i) = 2^(i-1) + 2^(i-1))
*/
    int[][] dp;
    int maxPow;

    public TreeAncestor(int n, int[] parent) {
        // log_base_2(n)
        maxPow = (int) (Math.log(n) / Math.log(2)) + 1;
        dp = new int[maxPow][n];
        dp[0] = parent;
        for (int i = 1; i < maxPow; i ++) {
            for (int j = 0; j < n; j ++) {
                int pre = dp[i - 1][j];
                dp[i][j] = pre == -1 ? -1 : dp[i - 1][pre];
            }
        }
    }

    public int getKthAncestor(int node, int k) {
        int maxPow = this.maxPow;
        while (node > -1 && k > 0) {
            if (k >= (1 << maxPow)) {
                node = dp[maxPow][node];
                k -= (1 << maxPow);
            } else {
                maxPow --;
            }
        }
        return node;
    }
}

/**
 * Your TreeAncestor object will be instantiated and called as such:
 * TreeAncestor obj = new TreeAncestor(n, parent);
 * int param_1 = obj.getKthAncestor(node,k);
 */