Lowest Common Ancestor of a Binary Tree IV

Medium

Given the root of a binary tree and an array of TreeNode objects nodes, return the lowest common ancestor (LCA) of all the nodes in nodes. All the nodes will exist in the tree, and all values of the tree’s nodes are unique.

Extending the definition of LCA on Wikipedia: “The lowest common ancestor of n nodes p1p2, …, pn in a binary tree T is the lowest node that has every pi as a descendant (where we allow a node to be a descendant of itself) for every valid i“. A descendant of a node x is a node y that is on the path from node x to some leaf node.

Example 1:

Binary Tree
Input: root = [3,5,1,6,2,0,8,null,null,7,4], nodes = [7,6,2,4]
Output: 5
Explanation: The lowest common ancestor of the nodes 7, 6, 2, and 4 is node 5.

Example 2:

Input: root = [3,5,1,6,2,0,8,null,null,7,4], nodes = [0,1,2,3,4,5,6,7,8]
Output: 3
Explanation: The lowest common ancestor of all the nodes is the root node.

Implementation

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    
    TreeNode lca = null;
    
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode[] nodes) {
        Set<Integer> targetNodes = new HashSet<>();
        for(TreeNode node : nodes) {
            targetNodes.add(node.val);
        }
        DFS(root, targetNodes);
        return lca;
    }
    
    private int DFS(TreeNode root, Set<Integer> nodes) {
        if(root == null) return 0;
        int leftCount = DFS(root.left, nodes);
        int rightCount = DFS(root.right, nodes);
        int foundCount = leftCount + rightCount;
        
        if(nodes.contains(root.val)) {
            foundCount++;
        }
        
        if(foundCount == nodes.size() && lca == null) {
            lca = root;
        }
        
        return foundCount;
    }
}

Categories: Binary Tree, Data Structure

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