算法练习第二十一天|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 {
    //双指针法
    TreeNode pre = null;
    int result = Integer.MAX_VALUE;
    public int getMinimumDifference(TreeNode root) {
        if(root == null) return 0;
        generateResult(root);
        return result;
    }

    public void generateResult(TreeNode root){
        if(root == null) return;
        generateResult(root.left);
        if(pre !=null){
            result = Math.min(result,root.val-pre.val);
        }
        pre = root;
        generateResult(root.right);
    }
}

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 {
    int count = 0;//定义频率；
    int maxCount = 0;//定义最大频率
    List<Integer> resultList = new ArrayList<>();
    TreeNode pre = null;
    public int[] findMode(TreeNode root) {
        doGenerateResult(root);
        int[] res = new int[resultList.size()];
        for(int i = 0;i<res.length;i++){
            res[i] = resultList.get(i);
        }
        return res;
    }

    public void doGenerateResult(TreeNode root){
        if(root == null) return;

        doGenerateResult(root.left);


        int val = root.val;
        if(pre == null || pre.val != root.val){
            count = 1;
        }else{
            count ++;
        }

        if(count > maxCount){
            maxCount = count;
            resultList.clear();
            resultList.add(val);
        }else if(count == maxCount){
            resultList.add(val);
        }
        pre = root;

        doGenerateResult(root.right);
    }
}

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) {
        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 null;
        }else if(left != null && right == null){
            //从左节点返回
            return left;
        }else if(left == null && right != null){
            // //从右节点返回
            return right;
        }else{
            //左右节点满足
            return root;
        }

    }
}