Lowest Common Ancestor of a BST

LeetCode #235


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).”


       /              \
    ___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.





class Solution {
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        if(root==p || root==q) return root; // p or q is the ancestor

        int min_val = min(p->val, q->val);
        int max_val = max(p->val, q->val);

        // 根据BST的性质,那就该是root
        if(root->val>min_val && root->val<max_val) return root;

        // root比两个都小,那应该在right subtree
        if(root->val < min_val) return lowestCommonAncestor(root->right, p, q);

        //root比两个都大,那应该在left subtree
        if(root->val > max_val) return lowestCommonAncestor(root->left, p, q);

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