LeetCode-236.Lowest Common Ancestor of a Binary Tree

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 {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int x) { val = x; }
 * }
 */

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);
        return left == null ? right : right == null ? left : root;
    }

关于LCA问题，这篇博客写的还不错： https://www.hrwhisper.me/algorithm-lowest-common-ancestor-of-a-binary-tree/