day22 打卡235. 二叉搜索树的最近公共祖先 701.二叉搜索树中的插入操作 450.删除二叉搜索树中的节点

235. 二叉搜索树的最近公共祖先

1.递归法。利用二叉搜素树中间节点肯定大于左子树，小于右子树的特征。

class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if (root == null) return root;

        if (root.val > p.val && root.val > q.val) {
            // 向左子树搜索
            TreeNode left = lowestCommonAncestor(root.left, p , q);
            if (left != null) return left;
        }
        if (root.val < p.val && root.val < q.val) {
            // 想右子树搜索
            TreeNode right = lowestCommonAncestor(root.right, p, q);
            if (right != null) return right;
        }
        return root;
    }
}

2.迭代法

class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        while (true) {
            if (root.val > p.val && root.val > q.val) {
                root = root.left;
            } else if (root.val < p.val && root.val < q.val) {
                root = root.right;
            } else {
                break;
            }
        }
        return root;
    }
}

701.二叉搜索树中的插入操作

701题目链接

1.迭代法

class Solution {
    public TreeNode insertIntoBST(TreeNode root, int val) {
        TreeNode add = new TreeNode(val);
        if (root == null) return add;
        TreeNode cur = root;
        TreeNode pre = null;
        while (true) {
            if (cur == null) break;
            pre = cur;
            if (cur.val > val) {
                cur = cur.left;
            } else if (cur.val < val) {
                cur = cur.right;
            }
        }
        if (pre.val > val) {
            pre.left = add;
        } else if (pre.val < val) {
            pre.right = add;
        }
        return root;
    }
}

2.递归法

class Solution {
    public TreeNode insertIntoBST(TreeNode root, int val) {
        if (root == null) return new TreeNode(val);;
        if (root.val > val) {
            root.left = insertIntoBST(root.left, val);
        }
        if (root.val < val) {
            root.right = insertIntoBST(root.right, val);
        }
        return root;
    }
}

450.删除二叉搜索树中的节点

450题目链接

1.递归法

class Solution {
    public TreeNode deleteNode(TreeNode root, int key) {
        if (root == null) return root;

        if (root.val == key) {
            if (root.left == null) {
                return root.right;
            } else if (root.right == null) {
                return root.left;
            } else {
                TreeNode cur = root.right;
                while (cur.left != null) {
                    cur = cur.left;
                }
                cur.left = root.left;
                root = root.right;
                return root;
            }
        }

        if (root.val > key) root.left = deleteNode(root.left, key);  
        if (root.val < key) root.right = deleteNode(root.right, key);   
        return root;
    }
}