【Lowest Common Ancestor of a Binary Search Tree】cpp

题目：

Given a binary search tree (BST), find the lowest common ancestor (LCA) of two given nodes in the BST.

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).”

        _______6______
       /              \
    ___2__          ___8__
   /      \        /      \
   0      _4       7       9
         /  \
         3   5

For example, the lowest common ancestor (LCA) of nodes 2 and 8 is 6. Another example is LCA of nodes 2 and 4 is 2, since a node can be a descendant of itself according to the LCA definition.

代码：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
            if ( !root ) return NULL;
            if ( p->val>q->val ) swap(p, q);
            if ( root->val>=p->val && root->val<=q->val )
            {
                return root;
            }
            else if ( root->val < p->val )
            {
                return Solution::lowestCommonAncestor(root->right, p, q);
            }
            else
            {
                return Solution::lowestCommonAncestor(root->left, p, q);
            }
    }
};

tips：

由于BST是数据有序的，因此找公共祖先要容易一些。

参考：http://bookshadow.com/weblog/2015/07/11/leetcode-lowest-common-ancestor-binary-search-tree/