Leetcode 235. 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.

递归版本：

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

迭代版本：

class Solution {
public:
TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
TreeNode* cur = root;
while (true) {
if (p -> val < cur -> val && q -> val < cur -> val)
cur = cur -> left;
else if (p -> val > cur -> val && q -> val > cur -> val)
cur = cur -> right;
else return cur;
}
}
};