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.

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     struct TreeNode *left;
 6  *     struct TreeNode *right;
 7  * };
 8  */
 9 struct TreeNode* lowestCommonAncestor(struct TreeNode* root, struct TreeNode* p, struct TreeNode* q) {
10     if(!root && !p && !q)
11       return 0;
12     if(root->val == p->val)
13       return p;
14     if(root->val == q->val)
15       return q;
16     if((root->val > p->val && root->val < q->val)||(root->val < p->val && root->val > q->val))
17       return root;
18     if(root->val > p->val && root->val > q->val)
19       return lowestCommonAncestor(root->left,p,q);
20     if(root->val < p->val && root->val < q->val)
21       return lowestCommonAncestor(root->right,p,q);
22     return 0;
23 }

 

posted @ 2015-07-23 12:24  鸭子船长  阅读(175)  评论(0)  编辑  收藏  举报