代码随想录 Day21 - 二叉树（七）

/**
 * 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 TreeNode pre;
    private int minDifference;
    public int getMinimumDifference(TreeNode root) {
        this.minDifference = Integer.MAX_VALUE;
        traversal(root);
        return minDifference;
    }
    private void traversal(TreeNode current) {
        if (current == null) {
            return;
        }
        traversal(current.left);
        if (pre != null) {
            minDifference = Math.min(minDifference, current.val - pre.val);
        }
        pre = current;
        traversal(current.right);
    }
}

/**
 * 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 List<Integer> result;
    private int maxCount;
    private int curCount;
    private TreeNode pre;
    public int[] findMode(TreeNode root) {
        this.result = new ArrayList<>();
        traversal(root);
        int[] res = new int[result.size()];
        for (int i = 0; i < result.size(); i++) {
            res[i] = result.get(i);
        }
        return res;
    }
    private void traversal(TreeNode current) {
        if (current == null) {
            return;
        }
        traversal(current.left);
        if (result.isEmpty() && maxCount == 0) {
            result.add(current.val);
        }
        int curVal = current.val;
        if (pre == null || curVal != pre.val) {
            curCount = 1;
        } else {
            curCount++;
        }
        if (curCount > maxCount) {
            result = new ArrayList<>();
            result.add(curVal);
            maxCount = curCount;
        } else if (curCount == maxCount) {
            result.add(curVal);
        }
        pre = current;
        traversal(current.right);
    }
}

/**
 * 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) { // 递归结束条件
            return root;
        }
        // 后序遍历
        TreeNode left = lowestCommonAncestor(root.left, p, q);
        TreeNode right = lowestCommonAncestor(root.right, p, q);
        if (left == null && right == null) { // 若未找到节点 p 或 q
            return null;
        } else if (left == null) { // 若找到一个节点
            return right;
        } else if (right == null) { // 若找到一个节点
            return left;
        } else { // 若找到两个节点
            return root;
        }
    }
}