二叉搜索树的最近公共祖先-力扣235

二叉搜索树的最近公共祖先-力扣235

用两个指针分别标记从根节点去往这两个结点路径，根据BST的性质，同时向下搜索。当两条路径出现分岔时，出现分岔的这个结点即为最近公共祖先。

/**
 * 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) {
        TreeNode* pPath, *qPath, *anscester;
        pPath = qPath = anscester = root;
        
        while(true){
            if(p->val < pPath->val) 
                pPath = pPath->left;
            else if(p->val > pPath->val) 
                pPath = pPath->right;

            if(q->val < qPath->val) 
                qPath = qPath->left;
            else if(q->val > qPath->val) 
                qPath = qPath->right;

            if(pPath == qPath) anscester = pPath;

            if(pPath != qPath) break;
        }
        

        return anscester;
    }
};

时间复杂度：平均$O(logn)$，即树高

空间复杂度：$O(1)$