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235-Lowest-Common-Ancestor-of-a-Binary-Search-Tree

问题描述

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 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).”

Example 1:

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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:

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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:

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Input: root = [2,1], p = 2, q = 1
Output: 2

Constraints:

  • The number of nodes in the tree is in the range [2, 105].
  • -109 <= Node.val <= 109
  • All Node.val are unique.
  • p != q
  • p and q will exist in the BST.

解题思路

  • 二叉搜索树,若是p,q比当前根结点小,则父节点必在左子树中,反之,则在右子树中。

  • 代码如下:

  • JavaScript

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var lowestCommonAncestor = function(root, p, q) {
while (root) {
if (root.val < p.val && root.val < q.val) {
root = root.right;
}
else if (root.val > p.val && root.val > q.val) {
root = root.left;
} else {
break;
}
}
return root;
};
  • 时间复杂度:O(n)
  • 空间复杂度:O(1)