173.Binary_Search_Tree_Iterator

Author: illuz

solutions/173.Binary_Search_Tree_Iterator/AC_pre_inorder_n.cpp

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+/*
+*  Author:      illuz <iilluzen[at]gmail.com>
+*  File:        AC_pre_inorder_n.cpp
+*  Create Date: 2015-03-16 10:07:47
+*  Descripton:   
+*/
+
+#include <bits/stdc++.h>
+
+using namespace std;
+const int N = 0;
+
+// Definition for binary tree
+struct TreeNode {
+    int val;
+    TreeNode *left;
+    TreeNode *right;
+    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
+};
+
+class BSTIterator {
+private:
+    queue<int> in_res;
+    void inorder(TreeNode *root) {
+        if (root != NULL) {
+            inorder(root->left);
+            in_res.push(root->val);
+            inorder(root->right);
+        }
+    }
+
+public:
+    BSTIterator(TreeNode *root) {
+        // generate the inorder list
+        while (!in_res.empty())
+            in_res.pop();
+        inorder(root);
+    }
+
+    /** @return whether we have a next smallest number */
+    bool hasNext() {
+        return in_res.size() != 0;
+    }
+
+    /** @return the next smallest number */
+    int next() {
+        int front = in_res.front();
+        in_res.pop();
+        return front;
+    }
+};
+
+int main() {
+    BSTIterator i = BSTIterator(root);
+    while (i.hasNext()) cout << i.next();
+    return 0;
+}
+
+

solutions/173.Binary_Search_Tree_Iterator/AC_stack_n.py

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+#!/usr/bin/python
+# -*- coding: utf-8 -*-
+# Author:      illuz <iilluzen[at]gmail.com>
+# File:        AC_stack_n.py
+# Create Date: 2015-03-16 10:19:51
+# Usage:       AC_stack_n.py 
+# Descripton:  
+
+
+# Definition for a  binary tree node
+class TreeNode:
+    def __init__(self, x):
+        self.val = x
+        self.left = None
+        self.right = None
+
+class BSTIterator:
+    # @param root, a binary search tree's root node
+    def __init__(self, root):
+        self.stack = []
+        while root:
+            self.stack.append(root)
+            root = root.left
+
+    # @return a boolean, whether we have a next smallest number
+    def hasNext(self):
+        return len(self.stack) != 0
+
+    # @return an integer, the next smallest number
+    def next(self):
+        top = self.stack.pop()
+        right = top.right
+        while right:
+            self.stack.append(right)
+            right = right.left
+        return top.val
+
+# debug
+root = TreeNode(0)
+i, v = BSTIterator(root), []
+while i.hasNext(): v.append(i.next())

solutions/173.Binary_Search_Tree_Iterator/readme.md

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+## 173. Binary Search Tree Iterator (Medium)
+
+### **链接**：
+题目：https://leetcode.com/problems/binary-search-tree-iterator/  
+代码(github)：https://github.com/illuz/leetcode
+
+### **题意**：
+实现一个 BST 的类。
+
+### **分析**：
+
+1. **(C++)** 初始化的时候就将中序遍历处理好，保存在 queue 里
+2. **(Python)** 用 stack 做，一边求一边处理即可。
+3. Morris Travese Tree，可以用 O(1) 的空间解决这题，详见 [099. Recover Binary Search Tree (Hard)](https://github.com/illuz/leetcode/blob/master/solutions/099.Recover_Binary_Search_Tree)