代码随想录训练营第三十期|第二十一天|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 ● 236. 二叉树的最近公共祖先

530. 二叉搜索树的最小绝对差 – 力扣（LeetCode）

/**
 * 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 int getMinimumDifference(TreeNode root) {
        List list = new ArrayList();
        if (root == null) return 0;
        traversal(root, list);
        int res = Integer.MAX_VALUE;
        for (int i = 1; i < list.size(); i++) {
            int dif = Math.abs(list.get(i) - list.get(i - 1));
            res = Math.min(dif, res);
        }
        return res;
    }

    private void traversal(TreeNode root, List list) {
        if (root == null) return;
        traversal(root.left, list);
        list.add(root.val);
        traversal(root.right, list);
    }
}

501. 二叉搜索树中的众数 – 力扣（LeetCode）

/**
 * 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 {
    int max = 0;
    Map map = new HashMap();
    public int[] findMode(TreeNode root) {
        List list = new ArrayList();
        traversal(root);
        for (Integer n : map.keySet()) {
            if (map.get(n) == max) {
                list.add(n);
            }
        }
        int[] res = new int[list.size()];
        for (int i = 0; i < list.size(); i++) {
            res[i] = list.get(i);
        }
        return res;
    }

    private void traversal(TreeNode root) {
        if (root == null) return;
        traversal(root.left);
        map.put(root.val, map.getOrDefault(root.val, 0) + 1);
        max = Math.max(max, map.get(root.val));
        traversal(root.right);
    }
}

236. 二叉树的最近公共祖先 – 力扣（LeetCode）

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
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 right;
        } else if (left != null && right == null) {
            return left;
        } else if (left == null && right == null) {
            return null;
        }
        return root;
    }
}

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