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530.二叉搜索树的最小绝对差

力扣题目链接：. - 力扣（LeetCode）

中序递归

class Solution {TreeNode pre;int res=Integer.MAX_VALUE;public int getMinimumDifference(TreeNode root) {midorder(root);return res;}public void midorder(TreeNode root){if(root==null){return;}midorder(root.left);if(pre!=null){res=Math.min(res,root.val-pre.val);}pre=root;midorder(root.right);}
}

501.二叉搜索树中的众数

力扣题目链接：. - 力扣（LeetCode）

中序递归

class Solution {int count=0;int maxcount=0;TreeNode pre=null;public int[] findMode(TreeNode root) {List<Integer> res=new ArrayList<>();midorder(root,res);int[] result=new int[res.size()];for(int i=0;i<res.size();i++){result[i]=res.get(i);}return result;}public void midorder(TreeNode root,List<Integer> res){if(root==null){return;}midorder(root.left,res);      if(pre==null||root.val!=pre.val){count=1;}else{count++;}if(count>maxcount){res.clear();res.add(root.val);maxcount=count;}else if(count==maxcount){res.add(root.val);}pre=root;midorder(root.right,res);}
}

236. 二叉树的最近公共祖先

力扣题目链接：. - 力扣（LeetCode）

class Solution {public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {if(root==null||root==p||root==q){return root;}TreeNode left=lowestCommonAncestor(root.left,p,q);TreeNode right=lowestCommonAncestor(root.right,p,q);if(left==null&&right==null){return null;}if(left!=null&&right==null){return left;}if(left==null&&right!=null){return right;}return root;}
}