LeetCode-Lowest Common Ancestor of a Binary Tre

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.

 

 Solution:

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

        public Result(boolean p, boolean q) {
            findP = p;
            findQ = q;
            ancestor = null;
        }
    }

    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        return findAncestorRecur(root, p, q).ancestor;
    }

    public Result findAncestorRecur(TreeNode cur, TreeNode p, TreeNode q) {
        if (cur == null) {
            return new Result(false, false);
        }

        boolean findP = (cur == p), findQ = (cur == q);
        Result leftRes = findAncestorRecur(cur.left, p, q);
        if (leftRes.ancestor != null)
            return leftRes;
        Result rightRes = findAncestorRecur(cur.right, p, q);
        if (rightRes.ancestor != null)
            return rightRes;

        findP = (findP || leftRes.findP || rightRes.findP);
        findQ = (findQ || leftRes.findQ || rightRes.findQ);

        Result res = new Result(findP, findQ);
        if (findP && findQ)
            res.ancestor = cur;

        return res;
    }
}