19th Day| 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

LeetCode 530 二叉搜索树的最小绝对差

java 复制代码

class Solution {
    int min = Integer.MAX_VALUE;
    TreeNode pre; 
    public int getMinimumDifference(TreeNode root) {
        traversal(root);
        return min;
    }

    public void traversal(TreeNode root){
        if(root == null) return;
        traversal(root.left);
        if(pre != null && (root.val - pre.val) < min){
            min = root.val - pre.val;
        }
        pre = root;
        traversal(root.right);

    }
}

LeetCode 501 二叉搜索树中的众数

题目链接:501.二叉搜索树中的众数

java 复制代码

class Solution {
    TreeNode pre;
    int maxTimes = 1;
    int currentTimes;
    List<Integer> list = new ArrayList<>();
    public int[] findMode(TreeNode root) {
        if(root == null) return new int[]{};
        traversal(root);
        int[] res = new int[list.size()];
        for(int i = 0; i < list.size(); i++){
            res[i] = list.get(i);
        }
        return res;
    }

    public void traversal(TreeNode root){
        if(root == null) return;
        traversal(root.left);
        if(pre == null || root.val != pre.val){
            currentTimes = 1;
        }
        if(pre != null && root.val == pre.val){
            currentTimes+=1;
        }
        if(currentTimes > maxTimes){
            list.clear();
            maxTimes = currentTimes;
            list.add(root.val);
        }else if(currentTimes == maxTimes){
            list.add(root.val);
        }
        pre = root;
        traversal(root.right);
    }
}

LeetCode 236 二叉树的最近公共祖先

题目链接:236.二叉树的最近公共祖先

java 复制代码

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