随想录训练营Day21 | 第六章 二叉树 part07 530.二叉搜索树的最小绝对差,501. 二叉搜索树中的众数

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530.二叉搜索树的最小绝对差

思路:

• 二叉搜索树上求最值，求差值，思考二叉搜索树的有序性
• 用中序遍历,想象成一个递增的有序数组
• 用pre来记录遍历的前一个节点

class Solution {
    TreeNode pre = null;
    int res = Integer.MAX_VALUE;
    public int getMinimumDifference(TreeNode root) {
        if(root == null){
            return 0;
        }
        getMinimumDifference(root.left);

        if(pre != null){
            res = Math.min(res, root.val - pre.val);
        }
        pre = root;

        getMinimumDifference(root.right);
        return res;
    }
}

501. 二叉搜索树中的众数

class Solution {
    int count;
    int maxCount;
    TreeNode pre;
    ArrayList<Integer> resList;

    public int[] findMode(TreeNode root) {
        resList = new ArrayList<>();
        count = 0;
        maxCount = Integer.MIN_VALUE;
        pre = null;
        find(root);
        int[] res = new int[resList.size()];
        for (int i = 0; i < resList.size(); i++) {
            res[i] = resList.get(i);
        }
        return res;
    }
    public void find(TreeNode root){
        if(root == null){
            return;
        }
        find(root.left);
        //计数
        int rootValue = root.val;
        if(pre == null || rootValue != pre.val){
            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;

        find(root.right);
    }
}