給定一棵二叉樹，找到兩個節點的最近公共父節點(LCA)。最近公共祖先是兩個節點的公共的祖先節點且具有最大深度。

樣例：
對於下面這棵二叉樹

4
/ \
3 7
/ \
5 6
LCA(3, 5) = 4

LCA(5, 6) = 7

LCA(6, 7) = 7

這道題筆者最近在筆試中也遇到過，相對比較簡單，代碼如下：

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */
class Solution {
public:
    /**
     * @param root: The root of the binary search tree.
     * @param A and B: two nodes in a Binary.
     * @return: Return the least common ancestor(LCA) of the two nodes.
     */
    TreeNode *lowestCommonAncestor(TreeNode *root, TreeNode *A, TreeNode *B) {
        if (root == NULL || A == root || B == root) return root;
        TreeNode *left = lowestCommonAncestor(root->left, A, B);
        TreeNode *right = lowestCommonAncestor(root->right, A, B);
        if (left != NULL && right != NULL) return root;
        return left ? left : right;
    }
};

二叉樹最近公共祖先