Problem 173: Binary Search Tree Iterator
思路
这道题的难点在于理解题目的意思:题目是说找到下一个最小的数,而不是判断下一个数在哪儿。否则如果是找下一个数的话,直接找就是当前结点的右边。
这道题相当用 DFS 求解,用一个 stack 来维护一个最小值,就是一直把最左边的结点装到 stack 里面,用 stack pop 下一个最小值出来
/**
* Definition for binary tree
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class BSTIterator {
private Stack<TreeNode> stack = new Stack<TreeNode>();
private TreeNode curr;
public BSTIterator(TreeNode root) {
this.curr = root;
}
/** @return whether we have a next smallest number */
public boolean hasNext() {
return curr != null || !stack.isEmpty();
}
/** @return the next smallest number */
public int next() {
while (curr != null) {
stack.push(curr);
curr = curr.left;
}
TreeNode node = stack.pop();
curr = node.right;
return node.val;
}
}
/**
* Your BSTIterator will be called like this:
* BSTIterator i = new BSTIterator(root);
* while (i.hasNext()) v[f()] = i.next();
*/
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