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前言

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像求和、求高度这种基本的二叉树函数很容易写，有时候只要在它们的后序位置添加一点代码，就能得到我们想要的答案。

一、力扣549. 二叉树中最长的连续序列

/*** 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 {int res = 0;public int longestConsecutive(TreeNode root) {fun(root);return res;}public int[] fun(TreeNode root){if(root == null){return new int[]{0,0};}int[] left = fun(root.left);int[] right = fun(root.right);int leftInc = left[0], leftDes = left[1];int rightInc = right[0], rightDes = right[1];int curInc = 1, curDes = 1;if(root.left != null){if(root.left.val -1 == root.val){curInc += leftInc;}if(root.left.val + 1 == root.val){curDes += leftDes;}}if(root.right != null){if(root.right.val - 1 == root.val){curInc = Math.max(curInc,rightInc+1);}if(root.right.val + 1 == root.val){curDes = Math.max(curDes, rightDes+1);}}res = Math.max(res, curInc + curDes -1);return new int[]{curInc,curDes};}
}

二、力扣1325. 删除给定值的叶子节点

/*** 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 {public TreeNode removeLeafNodes(TreeNode root, int target) {if(root == null){return null;}if(root.left == null && root.right == null){if(root.val == target){return null;}}TreeNode left = removeLeafNodes(root.left,target);TreeNode right = removeLeafNodes(root.right,target);if(left == null && right == null){if(root.val == target){return null;}}root.left = left;root.right = right;return root;}
}