LeetCode-in-Java

1123. Lowest Common Ancestor of Deepest Leaves

Medium

Given the root of a binary tree, return the lowest common ancestor of its deepest leaves.

Recall that:

• The node of a binary tree is a leaf if and only if it has no children
• The depth of the root of the tree is 0. if the depth of a node is d, the depth of each of its children is d + 1.
• The lowest common ancestor of a set S of nodes, is the node A with the largest depth such that every node in S is in the subtree with root A.

Example 1:

Input: root = [3,5,1,6,2,0,8,null,null,7,4]

Output: [2,7,4]

Explanation: We return the node with value 2, colored in yellow in the diagram. The nodes coloured in blue are the deepest leaf-nodes of the tree. Note that nodes 6, 0, and 8 are also leaf nodes, but the depth of them is 2, but the depth of nodes 7 and 4 is 3.

Example 2:

Input: root = [1]

Output: [1]

Explanation: The root is the deepest node in the tree, and it’s the lca of itself.

Example 3:

Input: root = [0,1,3,null,2]

Output: [2]

Explanation: The deepest leaf node in the tree is 2, the lca of one node is itself.

Constraints:

• The number of nodes in the tree will be in the range [1, 1000].
• 0 <= Node.val <= 1000
• The values of the nodes in the tree are unique.

Note: This question is the same as 865: https://leetcode.com/problems/smallest-subtree-with-all-the-deepest-nodes/

Solution

import com_github_leetcode.TreeNode;

/*
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
public class Solution {
    public TreeNode lcaDeepestLeaves(TreeNode root) {
        if (root == null) {
            return null;
        }
        int leftDep = getDep(root.left);
        int rightDep = getDep(root.right);
        if (leftDep == rightDep) {
            return root;
        } else {
            if (leftDep > rightDep) {
                return lcaDeepestLeaves(root.left);
            } else {
                return lcaDeepestLeaves(root.right);
            }
        }
    }

    public int getDep(TreeNode root) {
        if (root == null) {
            return 0;
        }
        return 1 + Math.max(getDep(root.left), getDep(root.right));
    }
}

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