LeetCode 235: Lowest Common Ancestor of a Binary Search Tree
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wiki的解释:二叉查找树,也称二叉搜索树、有序二叉树,排序二叉树,是指一棵空树或者具有下列性质的二叉树:
1.若任意节点的左子树不空,则左子树上所有结点的值均小于或等于它的根结点的值;
2.任意节点的右子树不空,则右子树上所有结点的值均大于它的根结点的值;
3.任意节点的左、右子树也分别为二叉查找树。没有键值相等的节点。
根据这种规则,可知:
1.如果root的值在p和q之间,root一定是Lowest Common Ancestor。
2.如果root比p、q都大,对root左边递归。
3.如果root比p、q都小,对root右边递归。
一直找中间值就能找出Lowest Common Ancestor。
/** * 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->val <= root->val && root->val <= q->val || p->val >= root->val && root->val >= q->val){ return root; } else if(p->val > root->val && q->val > root->val){ return lowestCommonAncestor(root->right , p , q); } else if(p->val < root->val && q->val < root->val){ return lowestCommonAncestor(root->left , p , q); } }};
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