173. Binary Search Tree Iterator

標籤：

建一個stack，最末的元素始終是最小的那個。

所以對於初始化，存到最左下角的那個節點。

對於找到next，就是存到，當前節點的右子節點的最左下角（如果有的話）。

 1 public class BSTIterator { 2     Stack<TreeNode> stack; 3      4     public BSTIterator(TreeNode root) { 5         stack = new Stack<TreeNode>(); 6         TreeNode cur = root; 7         while(cur != null) { 8             stack.push(cur); 9             cur = cur.left;10         }11     }12 13     /** @return whether we have a next smallest number */14     public boolean hasNext() {15         return !stack.isEmpty();16     }17 18     /** @return the next smallest number */19     public int next() {20         TreeNode cur = stack.pop();21         int next = cur.val;22         cur = cur.right;23         while(cur != null) {24             stack.push(cur);25             cur = cur.left;26         }27         return next;28     }29 }

 

173. Binary Search Tree Iterator