Leet Code 第173题Binary Search Tree Iterator 解题思路
1、读题，题目要求为BST 实现一个迭代器，有二个方法，next 和hasNext 方法。
2、先按照中序遍历，变成一个数组或list等，再按照索引取值。
3、该题关键点是索引的边界条件。
下面黄哥用Python、Go、Java 三种语言实现的代码。
Python 代码
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
# 黄哥Python培训 黄哥所写
class BSTIterator:
def __init__(self, root: TreeNode):
res = []
def dfs(root):
if root is None:
return
dfs(root.left)
res.append(root.val)
dfs(root.right)
dfs(root)
# print(res)
self.nodes = res
self.index = -1
def next(self) -> int:
"""
@return the next smallest number
"""
self.index += 1
if self.index < len(self.nodes):
return self.nodes[self.index]Go 代码
/**
* Definition for a binary tree node.
* type TreeNode struct {
* Val int
* Left *TreeNode
* Right *TreeNode
* }
*/
// 黄哥Python培训 黄哥所写
type BSTIterator struct {
nodes []int
index int
}
func Constructor(root *TreeNode) BSTIterator {
if root == nil {
return BSTIterator{}
}
res := BSTIterator{
index: -1,
}
dfs(root, &res)
return res
}
func dfs(root *TreeNode, res *BSTIterator) {
if root == nil {
return
}
dfs(root.Left, res)
res.nodes = append(res.nodes, root.Val)
dfs(root.Right, res)
}
/** @return the next smallest number */
func (this *BSTIterator) Next() int {
// fmt.Println(this.nodes)
this.index += 1
if this.index < len(this.nodes) {
return this.nodes[this.index]
}
this.index = -1
return this.nodes[this.index]
}
/** @return whether we have a next smallest number */
func (this *BSTIterator) HasNext() bool {
if this.index == len(this.nodes) - 1 {
this.index += 1
}
if this.index >= -1 && this.index <= len(this.nodes) - 1 {
return true
}
return false
}
/**
* Your BSTIterator object will be instantiated and called as such:
* obj := Constructor(root);
* param_1 := obj.Next();
* param_2 := obj.HasNext();
*/Java 代码
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
// 黄哥Python培训 黄哥所写
class BSTIterator {
List<Integer> nodes = new ArrayList<>();
int index;
public BSTIterator(TreeNode root) {
dfs(root);
index = -1;
}
public void dfs(TreeNode root) {
if (root == null) {
return;
}
dfs(root.left);
nodes.add(root.val);
dfs(root.right);
}
/** @return the next smallest number */
public int next() {
index += 1;
if (index < nodes.size() ) {
// return nodes[index];
return nodes.get(index);
}
index = -1;
return nodes.get(index);
}
/** @return whether we have a next smallest number */
public boolean hasNext() {
if (index == nodes.size() - 1) {
index += 1;
}
if (index >= -1 && index <= nodes.size() - 1) {
return true;
}
return false;
}
}
/**
* Your BSTIterator object will be instantiated and called as such:
* BSTIterator obj = new BSTIterator(root);
* int param_1 = obj.next();
* boolean param_2 = obj.hasNext();
*/
黄哥：黄哥Python培训是这样训练学员的zhuanlan.zhihu.com
