【LeetCode】235. Lowest Common Ancestor of a Binary Search Tree 二叉搜尋樹的最近公共祖先(Easy)(JAVA)
阿新 • • 發佈:2020-12-14
技術標籤:Leetcode演算法java二叉樹leetcode資料結構
【LeetCode】235. Lowest Common Ancestor of a Binary Search Tree 二叉搜尋樹的最近公共祖先(Easy)(JAVA)
題目地址: https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-search-tree/
題目描述:
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 p and qas the lowest node in T that has both p and qas descendants (where we allow a node to be a descendant of itself).”
Example 1:
![](https://img.796t.com/res/2020/12-14/19/23eabd5bbcca99ebed163bfa851bfb61.png)
Input: root = [6,2,8,0,4,7,9,null,null,3,5], p = 2, q = 8 Output: 6 Explanation: The LCA of nodes 2 and 8 is 6.
Example 2:
![](https://img.796t.com/res/2020/12-14/19/23eabd5bbcca99ebed163bfa851bfb61.png)
Input: root = [6,2,8,0,4,7,9,null,null,3,5], p = 2, q = 4
Output: 2
Explanation: The LCA of nodes 2 and 4 is 2, since a node can be a descendant of itself according to the LCA definition.
Example 3:
Input: root = [2,1], p = 2, q = 1
Output: 2
Constraints:
- The number of nodes in the tree is in the range [2, 10^5].
- -10^9 <= Node.val <= 10^9
- All Node.val are unique.
- p != q
- p and q willexist in the BST.
題目大意
給定一個二叉搜尋樹, 找到該樹中兩個指定節點的最近公共祖先。
百度百科中最近公共祖先的定義為:“對於有根樹 T 的兩個結點 p、q,最近公共祖先表示為一個結點 x,滿足 x 是 p、q 的祖先且 x 的深度儘可能大(一個節點也可以是它自己的祖先)。”
解題方法
- 因為是二叉搜尋樹,滿足左子樹都比當前節點小,右子樹都比當前節點大
- 最近的祖先節點:滿足 p,q 中的較小值在左邊,較大值在右邊(都是大於等於的情況)
- 如果不滿足,但是當前節點比最大值還大,那肯定在左子樹
- 如果當前節點比最小值還小,肯定在右子樹
class Solution {
public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
if (p.val > q.val) {
TreeNode temp = p;
p = q;
q = temp;
}
while (root != null) {
if (root.val >= p.val && root.val <= q.val) return root;
if (root.val > p.val) {
root = root.left;
} else {
root = root.right;
}
}
return null;
}
}
執行耗時:7 ms,擊敗了38.20% 的Java使用者
記憶體消耗:39.6 MB,擊敗了62.45% 的Java使用者
![](https://img.796t.com/res/2020/12-09/16/b6631381964e83c42e084ea5a505572e.jpg)