二叉树的最近公共祖先

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
//递归 回溯
class Solution {
    TreeNode res=null;
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        dfs(root,p,q);
        return res;
        
    }
    private int dfs(TreeNode root,TreeNode p, TreeNode q){
        if(root==null)return 0;
        int left=dfs(root.left,p,q);
        int right=dfs(root.right,p,q);
        int mid=root==p||root==q?1:0;
        if(left+right+mid>1)res=root;
        return left+right+mid>0?1:0; 
    }
}