代码随想录算法训练营二十一天 | 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

代码随想录算法训练营二十一天 | 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 node = null;
    int min = Integer.MAX_VALUE;
    public int getMinimumDifference(TreeNode root) {
        //二叉搜索树
        // 中序遍历
        if (root == null) {
            return 0;
        }
        test(root);
        return min;
    }

    public int test(TreeNode root) {
        if (root == null) {
            return min;
        }
        int min1 = test(root.left);
        
        if (node != null && Math.abs(node.val - root.val) < min) {
            min = Math.abs(node.val - root.val);
        }
        node = root;
        
        int min2 = test(root.right);
        min = min1 < min2 ? min1 : min2;
        return min;
    }

}

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 {
    private int maxNum = 0; // 最大频率
    private int num = 0; // 统计频率
    private TreeNode node = null; // 记录上一个
    public int[] findMode(TreeNode root) {
        // 二叉搜索树就是不一样。遇到二叉搜索树就要记得使用中序遍历
        List<Integer> result = new ArrayList<>();
        if (root == null) {
            return result.stream().mapToInt(i->i).toArray();
        }
        test(root, result);
        return result.stream().mapToInt(i->i).toArray();
    }

    public void test (TreeNode root, List<Integer> result) {
        if (root == null) {
            return;
        }
        test(root.left, result);
        if (node == null) {
            // 第一次进来
            num++;
        } else if (node.val == root.val) {
            num++;
        } else {
            num = 1;
        }
        node = root;
        if (num == maxNum) {
            result.add(root.val);
        }
        if (num > maxNum) {
            maxNum = num;
            result.clear();
            result.add(root.val);
        }
        test(root.right, result);
    }
}

236. 二叉树的最近公共祖先

/**
 * 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) {
        // 还是学习思想,第一行代码就有很多逻辑
        // 采用后序遍历去处理
        if (root == p || root == q || root == null) {
            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 left;
        } else if (left == null && right != null) {
            return right;
        } else {
            return null;
        }
    }
}

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