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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;}
}