leetcode-lowest-common-ancestor-of-a-binary-tree

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题目大意

　　https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-tree/

　　二叉树最近邻公共祖先问题

题目分析

　　利用递归即可

代码

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/**
 * 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) {
        if (root == null || p == null || q == null) {
            return null;
        }
        if (p == root || q == root) {
            return root;
        }
        TreeNode l = lowestCommonAncestor(root.left, p, q);
        TreeNode r = lowestCommonAncestor(root.right, p, q);
        if (l != null && r != null) {
            return root;
        }
        if (l == null) {
            return r;
        }
        return l;
    }
}

　　时间复杂度O(n)，n是树的节点个数