lowestCommonAncestorOfABinaryTree.java

// Given a binary tree, find the lowest common ancestor (LCA) of two given nodes in the tree.

// According to the definition of LCA on Wikipedia: “The lowest common ancestor is defined between two nodes v and w as the lowest node in T that has both v and w as descendants (where we allow a node to be a descendant of itself).”

//         _______3______
//        /              \
//     ___5__          ___1__
//    /      \        /      \
//    6      _2       0       8
//          /  \
//          7   4
// For example, the lowest common ancestor (LCA) of nodes 5 and 1 is 3. Another example is LCA of nodes 5 and 4 is 5, since a node can be a descendant of itself according to the LCA definition.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {

    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        
        if(root == null || root == p || root == q) return root;
        
        TreeNode left = lowestCommonAncestor(root.left, p, q);
        TreeNode right = lowestCommonAncestor(root.right, p, q);
        
        if(left != null && right != null) return root;
        
        return left == null ? right : left;
        
    }
}