代码随想录三刷day22

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前言

遇到在二叉搜索树上求什么最值，求差值之类的，都要思考一下二叉搜索树可是有序的，要利用好这一特点。

一、力扣617. 合并二叉树

/**
 * 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 {
    public TreeNode mergeTrees(TreeNode root1, TreeNode root2) {
        if(root1 == null && root2 == null){
            return null;
        }
        if(root1 == null && root2 != null){
            return root2;
        }
        if(root1 != null && root2 == null){
            return root1;
        }
        root1.val = root1.val + root2.val;
        root1.left = mergeTrees(root1.left, root2.left);
        root1.right = mergeTrees(root1.right, root2.right);
        return root1;
    }
}

二、力扣700. 二叉搜索树中的搜索

/**
 * 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 {
    TreeNode res = null;
    public TreeNode searchBST(TreeNode root, int val) {
        fun(root,val);
        return res;
    }
    public void fun(TreeNode root, int val){
        if(root == null){
            return;
        }
        if(val == root.val){
            res = root;
            return;
        }else if(val > root.val){
            fun(root.right,val);
        }else{
            fun(root.left,val);
        }
    }
}

三、力扣98. 验证二叉搜索树

/**
 * 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<Integer> traverse = new ArrayList<>();
    public boolean isValidBST(TreeNode root) {
        fun(root);
        for(int i = 1; i < traverse.size(); i ++){
            // System.out.println(traverse.get(i));
            if(traverse.get(i) <= traverse.get(i-1)){
                return false;
            }
        }
        return true;
    }
    public void fun(TreeNode root){
        if(root == null){
            return;
        }
        fun(root.left);
        traverse.add(root.val);
        fun(root.right);
    }
}

四、力扣530. 二叉搜索树的最小绝对差

在这里插入代码片/**
 * 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<Integer> traverse = new ArrayList<>();
    public int getMinimumDifference(TreeNode root) {
        fun(root);
        int res = Integer.MAX_VALUE;
        for(int i = 1; i < traverse.size(); i ++){
            res = Math.min(res,traverse.get(i)-traverse.get(i-1));
        }
        return res;
    }
    public void fun(TreeNode root){
        if(root == null){
            return;
        }
        fun(root.left);
        traverse.add(root.val);
        fun(root.right);
    }
}