LCAOfBT.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 p and q as the lowest node in T that has both p and q as descendants (where we allow a node to be a descendant of itself).”

Example 1:
Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 1
Output: 3
Explanation: The LCA of nodes 5 and 1 is 3.
 */
public class LCAOfBT {
    public static void main(String[] args) {
        TreeNode root = new TreeNode(3);
        root.left=new TreeNode(5);
        root.right=new TreeNode(1);
        root.left.left=new TreeNode(6);
        root.left.right=new TreeNode(2);
        root.right.left=new TreeNode(0);
        root.right.right=new TreeNode(8);
        root.left.left.left=null;
        root.left.left.right=null;
        root.right.right.left=new TreeNode(7);
        root.right.right.right=new TreeNode(4);
        TreeNode ans=lowestCommonAncestor(root, new TreeNode(5),new TreeNode(1));
        System.out.println(ans.data);
    }
    public static 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;
        }
        if (left!=null){
            return left;
        }else{
            return right;
        }
    }
}