二叉树最小绝对差

双指针法

      private TreeNode pre;
public int getMinimumDifference(TreeNode root) {
    if(root == null) {
        return Integer.MAX_VALUE;
    }
    int a = getMinimumDifference(root.left);
    int c = Integer.MAX_VALUE;
    if(pre != null) {
        c = root.val - pre.val;
    } 
    pre = root;
    
    int b = getMinimumDifference(root.right);
    return Math.min(c,Math.min(a,b));
}

二叉搜索树中的众数

    private List<Integer> res = new ArrayList();
private int count;
private int maxCount;
private TreeNode pre;
public int[] findMode(TreeNode root) {
    find(root);
     return res.stream().mapToInt(Integer::intValue).toArray();
}

private void find(TreeNode root) {
    if(root == null) {
        return;
    }
    find(root.left);
    if(pre == null) {
        count = 1;
    } else if (pre.val == root.val) {
        count++;
    } else {
        count=1;
    }
    pre = root;

    if(maxCount == count) {
        res.add(root.val);
    } else if(maxCount < count) {
        res.clear();
        res.add(root.val);
        maxCount = count;

    }
    find(root.right);
}

二叉树最近公共祖先

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