力扣labuladong——一刷day33

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前言

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前序位置的代码只能从函数参数中获取父节点传递来的数据，而后序位置的代码不仅可以获取参数数据，还可以获取到子树通过函数返回值传递回来的数据。

一、力扣652. 寻找重复的子树

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    List<TreeNode> res;
    Map<String,Integer> invild;
    public List<TreeNode> findDuplicateSubtrees(TreeNode root) {
        res = new LinkedList<>();
        invild = new HashMap<>();
        fun(root);
        return res;
    }
    public String fun(TreeNode root){
        if(root == null){
            return "#";
        }
        String le = fun(root.left);
        String ri = fun(root.right);
        String cur = le + "," + ri + "," + root.val;
        if(invild.getOrDefault(cur,0) == 1){
            res.add(root);
        }
        invild.put(cur,invild.getOrDefault(cur,0)+1);
        return cur;
    }
}