Lowest Common Ancestor of a Binary Search Tree
Total Accepted: 41013 Total Submissions: 108082 Difficulty: Easy
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：
由於是二叉搜索樹。因此
1).當p,q值都比root大時，那最小公共祖先就在root的右子樹裏
2).當p,q值都比root小時，那最小公共祖先就在root的左子樹裏
3).其他情況下，最小公共祖先就是root這個節點。
/**
* 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->val==p->val||root->val==q->val){
return root;
}else if(root->val<p->val&&root->val<q->val){
return lowestCommonAncestor(root->right,p,q);
}else if(root->val>p->val&&root->val>q->val){
return lowestCommonAncestor(root->left,p,q);
}else{
return root;
}
}
};