Lowest Common Ancestor(最近公共祖先)

问题

Given the root and two nodes in a Binary Tree. Find the lowest common ancestor(LCA) of the two nodes.

The lowest common ancestor is the node with largest depth which is the ancestor of both nodes.

Notice

Assume two nodes are exist in tree.

Have you met this question in a real interview? Yes
Example
For the following binary tree:

LCA(3, 5) = 4

LCA(5, 6) = 7

LCA(6, 7) = 7

代码

/**
 * Definition of TreeNode:
 * public class TreeNode {
 * public int val;
 * public TreeNode left, right;
 * public TreeNode(int val) {
 * this.val = val;
 * this.left = this.right = null;
 * }
 * }
 */
public class Solution {
    /*
     * @param root: The root of the binary search tree.
     * @param A: A TreeNode in a Binary.
     * @param B: A TreeNode in a Binary.
     * @return: Return the least common ancestor(LCA) of the two nodes.
     */
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode A, TreeNode B) {
        // write your code here
        if (root == null || root == A || root == B) {
            return root;
        }
        TreeNode left = lowestCommonAncestor(root.left, A, B);
        TreeNode right = lowestCommonAncestor(root.right, A, B);
        if (left == null) {
            if (right == null) {
                return null;
            }
            return right;
        }
        if (right == null) {
            return left;
        }
        return root;
    }
}