https://leetcode.com/submissions/detail/32662938/
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.
Leetcode又出新题啦
SOLUTION 1：
使用递归可以轻松解决此问题。对于此题我们可以分为三种情况讨论：
1. P, Q都比root小，则LCA在左树，我们继续在左树中寻找LCA
2. P, Q都比root大，则LCA在右树，我们继续在右树中寻找LCA
3. 其它情况，表示P,Q在root两边，或者二者其一是root，或者都是root，这些情况表示root就是LCA，直接返回root即可。
代码如下：
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
// 1439.
// 1. Two nodes are on left, go left.
// 2. Two nodes are on right, go right.
// 3. They are in both sides, return root.
// if root == null, return null.
if (root == null) {
return null;
} if (p.val < root.val && q.val < root.val) {
return lowestCommonAncestor(root.left, p, q);
} else if (p.val > root.val && q.val > root.val) {
return lowestCommonAncestor(root.right, p, q);
} else {
return root;
}
}
}