public static void flattenIterative1(TreeNode root) {
Stack<TreeNode> stack = new Stack<>();
while (root != null) {
if (root.left != null) {
stack.push(root);
root = root.left;
} else if (root.right != null) {
stack.push(root);
root = root.right;
} else if (!stack.isEmpty()) {
TreeNode end = root;
while (!stack.isEmpty()
&& (stack.peek().left == null
|| (stack.peek().left != null && stack.peek().left != root))) {
root = stack.pop();
}
if (stack.isEmpty()) root = root.right;
else {
TreeNode top = stack.peek();
TreeNode temp = top.right;
top.right = root;
top.left = null;
end.right = temp;
root = (end.left != null || temp == null) ? end : temp;
}
} else root = root.right;
}
}
public static void flatten1(TreeNode root) {
while (root != null) {
if (root.left != null) flatten1(root.left);
TreeNode temp = root.right;
root.right = root.left;
root.left = null;
while (root.right != null) root = root.right;
root.right = temp;
root = root.right;
}
}
public static void flatten(TreeNode root) {
if (root == null) return;
flatten(root.right);
flatten(root.left);
root.right = prev;
root.left = null;
prev = root;
}
public static void flattenIterative(TreeNode root) {
if (root == null) return;
Stack<TreeNode> stack = new Stack<>();
stack.push(root);
while (!stack.isEmpty()) {
TreeNode treeNode = stack.pop();
if (treeNode.right != null) stack.push(treeNode.right);
if (treeNode.left != null) stack.push(treeNode.left);
if (!stack.isEmpty()) treeNode.right = stack.peek();
treeNode.left = null;
}
}