48 lines
1.6 KiB
Java
48 lines
1.6 KiB
Java
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package tree;
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import java.util.HashMap;
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import java.util.HashSet;
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/**
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* 题目： 236. 二叉树的最近公共祖先 (lowestCommonAncestor)
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* 描述：给定一个二叉树, 找到该树中两个指定节点的最近公共祖先。
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* 百度百科中最近公共祖先的定义为：“对于有根树 T 的两个节点 p、q，最近公共祖先表示为一个节点 x，满足 x 是 p、q 的祖先且 x 的深度尽可能大（一个节点也可以是它自己的祖先）。”
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* 链接：https://leetcode.cn/problems/lowest-common-ancestor-of-a-binary-tree/
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*/
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public class LowestCommonAncestor {
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void helper(HashMap<TreeNode, TreeNode> map, TreeNode father, TreeNode cur) {
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if (cur == null)
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return;
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map.put(cur, father);
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helper(map, cur, cur.left);
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helper(map, cur, cur.right);
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}
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public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
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HashMap<TreeNode, TreeNode> map = new HashMap<>();
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HashSet<TreeNode> set = new HashSet<>();
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map.put(root, null);
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helper(map, root, root.left);
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helper(map, root, root.right);
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// 将 p 以及其所有祖先加入 set
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TreeNode tp = p;
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while (tp != null) {
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set.add(tp);
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tp = map.get(tp);
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}
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// 从 q 开始，找到第一个出现在 p 祖先链中的节点
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tp = q;
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while (tp != null) {
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if (set.contains(tp))
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return tp;
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tp = map.get(tp);
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}
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return null; // 理论上不可能执行到这里，因为肯定存在公共祖先
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}
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}
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