LeetCode:Lowest Common Ancestor of a Binary Search Tree(BST最近公共祖先)

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.

思路：BST二叉查找树的LCA情况

查找两个node的最早的公共祖先，分三种情况：
1. 如果两个node在root的两边，那么最早的公共祖先就是root。
2. 如果两个node在root的左边，那么把root.leftChild作为root，再递归。
3. 如果两个node在root的右边，那么把root.rightChild作为root，再递归。

/**
* 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(!p||!q||!root) return NULL;

if(max(p->val,q->val)<root->val)
return lowestCommonAncestor(root->left,p,q);
else if(min(p->val,q->val)>root->val)
return lowestCommonAncestor(root->right,p,q);
else return root;

}
};