450.删除二叉搜索树中的节点

private void swapValue(TreeNode a, TreeNode b) {int t = a.val;a.val = b.val;b.val = t;
}public TreeNode deleteNode(TreeNode root, int key) {if (root == null) {return null;}if (key < root.val) {root.left = deleteNode(root.left, key);} else if (key > root.val) {root.right = deleteNode(root.right, key);} else {//case 4.case4又分为4种小情况，最后一种可以被合并掉。//所以这里只处理了三种。// 当前树只有一个结点，那么直接返回nullif (root.left == null && root.right == null) {return null;} else if (root.left != null) {// 当前结点还有左子树// 那么需要从左子树中找个较大的值。TreeNode large = root.left;while (large.right != null) {large = large.right;}//交换再删除swapValue(root, large);root.left = deleteNode(root.left, key);} else if (root.right != null) {// 当前结点还有右子树TreeNode small = root.right;//那么需要从右子树中找个较小的值while (small.left != null) {small = small.left;}//交换再删除swapValue(root, small);root.right = deleteNode(root.right, key);}}return root;
}