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README.md
tree.cpp

README.md

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.

Solution

由于是BST,若p,q节点都小于root,则一定在左子树,若p,q都大于root,则一定在右子树,否则一个大于root,一个小于root,则root就是最低祖先节点

class Solution {
public:
	TreeNode *lowestCommonAncestor(TreeNode *root, TreeNode *p, TreeNode *q) {
		if (root == nullptr)
			return nullptr;
		return lowestCommonAncestor(root, p->val, q->val);
	}
private:
	TreeNode *lowestCommonAncestor(TreeNode *root, int v1, int v2) {
		if (root->val > v1 && root->val > v2)
			return lowestCommonAncestor(root->left, v1, v2);
		if (root->val < v1 && root->val < v2)
			return lowestCommonAncestor(root->right, v1, v2);
		return root;
	}
};

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