530.二叉搜索树的最小绝对差

思路

使用中序遍历二叉搜索数，得到的结果集是有序递增的数组，可以用双指针法在有序递增的数组中找最小绝对差

class Solution {
    TreeNode pre = null;
    TreeNode min =null;
    public int getMinimumDifference(TreeNode root) {
        getMin(root);
        return min.val;
        
    }
    public void getMin(TreeNode root) {
        if(root== null){
            return ;
        }
        getMin(root.left);
        
        if(pre!=null){
            int diff = Math.abs(root.val-pre.val);
            if(min == null){
                min = new TreeNode(diff);
            }else if(diff < min.val){
                min.val = diff;
            }
        }
        pre = root;
   
        getMin(root.right);

        
    }
}

501.二叉搜索树中的众数

思路

• 使用后序遍历二叉搜索树，得到有序的数组，再用滑动窗口寻找元素出现的频率，并记录到结果集中。
• 注意：只要找到出现频率更高的元素，就清空之前收集的结果集

class Solution {

    TreeNode slow = null;
    int sum = 0;
    int max = 0;
    List<Integer> list = new ArrayList();
    public int[] findMode(TreeNode root) {
        getMaxNum(root);
        int[] nums = new int[list.size()];
        for(int i = 0 ; i < list.size() ; i++){
            nums[i] = list.get(i);
        }
        return nums;
    }
    public void getMaxNum(TreeNode root) {
        if(root == null){
            return ;
        }
        getMaxNum(root.left);
        if(slow == null){
            slow = root;
            sum = 1;
            max = sum;
            list.add(root.val);
        }else{
            if(slow.val!=root.val){
                slow = root;
                sum = 1;
                if(sum > max){
                    max = sum;
                    list.clear();
                    list.add(slow.val);
                }else if(sum == max){
                    list.add(slow.val);
                }
            }else if(slow.val == root.val){
                sum+=1;
                if(sum > max){
                    max = sum;
                    list.clear();
                    list.add(slow.val);
                }else if(sum == max){
                    list.add(slow.val);
                }
            }
        }
        getMaxNum(root.right);
    }
    public void delete(List<Integer> list){
        for (int i: list){
            list.remove(i);
        }
    }

}

236. 二叉树的最近公共祖先

思路

具体思路

class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        return getCommonAncestor(root, p, q);

    }
    public TreeNode getCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if(root == null){
            return null;
        }
        if(root == p || root == q){
            return root;
        }
        TreeNode left =  getCommonAncestor(root.left, p, q);
        TreeNode right = getCommonAncestor(root.right, p, q);
        if(left != null && right!= null){
            return root;
        }else if(left == null && right!= null){
            return right;
        }else if(left != null && right== null){
            return left;
        }else{
            return null;
        }

    }
}