readme.md

Leetcode

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 q as the lowest node in T that has both p and q as descendants (where we allow a node to be a descendant of itself).”

Given binary search tree:  root = [6,2,8,0,4,7,9,null,null,3,5]

Example 1:

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:

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.

Note:

All of the nodes' values will be unique. p and q are different and both values will exist in the BST.

Solution

类似于二分搜索. 公共祖先一定满足: 值在两个之间.

package leetcode235_LowestCommonAncestorOfABinarySearchTree;

import utils.TreeNode;

/**
 * Definition for a binary tree node. public class TreeNode { int val; TreeNode left; TreeNode
 * right; TreeNode(int x) { val = x; } }
 */
class Solution {
  public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
    if (root == null) return null;

    TreeNode current = root;
    int larger = Math.max(p.val, q.val);
    int smaller = Math.min(p.val, q.val);

    while (current != null) {
      if (current.val >= smaller && current.val <= larger) {
        return current;
      }

      if (current.val >= larger) {
        current = current.left;
        continue;
      }

      if (current.val <= smaller) {
        current = current.right;
      }
    }

    return current;
  }
}