LeetCode235:Lowest Common Ancestor of a Binary Search Tree

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.

二叉搜索树的最低公共祖先节点。由于二叉搜索树的特性，即任何一个节点的左子树中的节点值都比该节点值小，任何一个节点的右子树中的节点值都比该节点值大。可以根据这个特性在二叉树中查找。

对于给定的两个节点指针p和q，先调整p和q,使p指向值较小的那个节点，q指向值较大的那个节点；

然后从根节点node开始遍历，如果q的值小于node的值，表示p和q都在node的左子树中，更新node为node->left；

如果p的值大于node的值，表示p和q都在node的右子树中，更新node为node->right；

否则表示找到的这个节点就是p和q的最低公共祖先节点。

runtime:40ms

/**
 * 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==NULL||p==NULL||q==NULL)
            return NULL;
            
            //使p保存值较小的节点，q保存值较大的节点
            if(p->val > q->val)
            {
                /*
                TreeNode *tmp=p;
                p=q;
                q=tmp;
                */
                //上面的代码可以直接写成下面的样子
                swap(p,q);
            }
            
            TreeNode *result=root;
            while(true)
            {
                if(q->val < result->val)
                    result=result->left;
                else if(p->val >result->val)
                    result=result->right;
                else
                    return result;
            }
    }
};