一刷 05/2022
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode() {}
* TreeNode(int val) { this.val = val; }
* TreeNode(int val, TreeNode left, TreeNode right) {
* this.val = val;
* this.left = left;
* this.right = right;
* }
* }
*/
class BSTIterator {
// add a array to keep record of all elements that have been visited
private Stack<TreeNode> stack;
private List<Integer> visited;
private int index;
public BSTIterator(TreeNode root) {
stack = new Stack<>();
visited = new ArrayList<>();
index = 0;
while (root != null) {
stack.push(root);
root = root.left;
}
}
public boolean hasNext() {
return !stack.isEmpty() || index < visited.size();
}
public int next() {
int val;
if (index < visited.size()) {
val = visited.get(index);
} else {
TreeNode curr = stack.pop();
val = curr.val;
visited.add(val);
curr = curr.right;
while (curr != null) {
stack.push(curr);
curr = curr.left;
}
}
index++;
return val;
}
public boolean hasPrev() {
// index is pointing to the next value
return index - 2 >= 0;
}
public int prev() {
return visited.get(--index - 1);
}
}
/**
* Your BSTIterator object will be instantiated and called as such:
* BSTIterator obj = new BSTIterator(root);
* boolean param_1 = obj.hasNext();
* int param_2 = obj.next();
* boolean param_3 = obj.hasPrev();
* int param_4 = obj.prev();
*/
