代码随想录算法训练营第二十一天 | 530.二叉搜索树的最小绝对差|501.二叉搜索树中的众数| 236. 二叉树的最近公共祖先

530.二叉搜索树的最小绝对差

/**
 * 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 pre; //前一个节点
    int max = Integer.MAX_VALUE;
    public int getMinimumDifference(TreeNode root) {
        if(root == null) return 0;
        traversal(root);
        return max;
    }

    public void traversal(TreeNode root){
        if(root == null) return;
        traversal(root.left); //左
        if(pre != null){  //中
            max = Math.min(max,root.val-pre.val);
        }
        pre = root;
        traversal(root.right); //右
    }
}

501.二叉搜索树中的众数

/**
 * 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 {
    ArrayList resList;
    int maxCount;
    int count;
    TreeNode pre; //前一个节点
    public int[] findMode(TreeNode root) {
        resList = new ArrayList<>();
        maxCount = 0;
        count = 0;
        pre = null;
        findMode1(root);
        int[] res = new int[resList.size()];
        for (int i = 0; i < resList.size(); i++) {
            res[i] = resList.get(i);
        }
        return res;
    }

    private void findMode1(TreeNode root){
        if (root == null) {
            return;
        }
        findMode1(root.left); //左


        int rootValue = root.val; //吉鲁当前节点
        if(pre == null || rootValue != pre.val){  //二叉搜索树的特点，当当前节点和前一个节点不同，count赋1
            count = 1;
        }else {
            count++;
        }
        
        // 更新结果以及maxCount
        if (count > maxCount) {
            resList.clear();
            resList.add(rootValue);
            maxCount = count;
        } else if (count == maxCount) {
            resList.add(rootValue);
        }
        pre = root;

        findMode1(root.right); //右

    }
}

236. 二叉树的最近公共祖先

后面补