From 6df017863f41c02430b656fa3333d3cbbfe48d18 Mon Sep 17 00:00:00 2001
From: colorbox <colorbox222@gmail.com>
Date: Sun, 2 Feb 2025 15:08:34 +0900
Subject: [PATCH] 108. Convert Sorted Array to Binary Search Tree

---
 108/step1.cpp   | 30 ++++++++++++++++++++++++++++++
 108/step2_1.cpp | 21 +++++++++++++++++++++
 108/step2_2.cpp | 23 +++++++++++++++++++++++
 108/step2_3.cpp | 30 ++++++++++++++++++++++++++++++
 108/step3.cpp   | 24 ++++++++++++++++++++++++
 5 files changed, 128 insertions(+)
 create mode 100644 108/step1.cpp
 create mode 100644 108/step2_1.cpp
 create mode 100644 108/step2_2.cpp
 create mode 100644 108/step2_3.cpp
 create mode 100644 108/step3.cpp

diff --git a/108/step1.cpp b/108/step1.cpp
new file mode 100644
index 0000000..b6ad429
--- /dev/null
+++ b/108/step1.cpp
@@ -0,0 +1,30 @@
+/*
+Solve Time : 23:54
+
+Time : O(N log N)
+Space : (N)
+
+方針はほぼ最初にできていたが、右側の再帰部分でミスってハマった。
+右半分をreverseして再帰しないといけないという勘違いをしていた。
+*/
+class Solution {
+public:
+  TreeNode* sortedArrayToBST(vector<int>& nums) {
+    if (nums.size() == 0) {
+      return nullptr;
+    }
+    if (nums.size() == 1) {
+      return new TreeNode(nums[0]);
+    }
+    int half_size = nums.size() / 2;
+    vector<int> left_nums;
+    for (int i = 0; i < half_size; ++i) {
+      left_nums.push_back(nums[i]);
+    }
+    vector<int> right_nums;
+    for (int i = half_size + 1; i < nums.size(); ++i) {
+      right_nums.push_back(nums[i]);
+    }
+    return new TreeNode(nums[half_size], sortedArrayToBST(left_nums), sortedArrayToBST(right_nums));
+  }
+};
diff --git a/108/step2_1.cpp b/108/step2_1.cpp
new file mode 100644
index 0000000..d6503ae
--- /dev/null
+++ b/108/step2_1.cpp
@@ -0,0 +1,21 @@
+/*
+Time : O(N log N)
+Space : O(N)
+
+nums.size==1の無駄な処理減らし、vectorのコピーをループを使わずに実行するようにした。
+*/
+class Solution {
+public:
+  TreeNode* sortedArrayToBST(vector<int>& nums) {
+    if (nums.empty()) {
+      return nullptr;
+    }
+    int center_index = nums.size() / 2;
+    auto node = new TreeNode(nums[center_index]);
+    auto left_nodes = vector<int>(nums.begin(), nums.begin() + center_index);
+    node->left = sortedArrayToBST(left_nodes);
+    auto right_nodes = vector<int>(nums.begin() + center_index + 1, nums.end());
+    node->right = sortedArrayToBST(right_nodes);
+    return node;
+  }
+};
diff --git a/108/step2_2.cpp b/108/step2_2.cpp
new file mode 100644
index 0000000..7a41598
--- /dev/null
+++ b/108/step2_2.cpp
@@ -0,0 +1,23 @@
+/*
+Time : O(N)
+Space : O(N)
+インデックスを使用してvectorのコピーをなくした。
+*/
+class Solution {
+ public:
+  TreeNode* sortedArrayToBST(vector<int>& nums) {
+    return sorted_array_to_bst_recursive(nums, 0, nums.size());
+  }
+
+ private:
+  TreeNode* sorted_array_to_bst_recursive(vector<int>& nums, int left, int right) {
+    if (left >= right) {
+      return nullptr;
+    }
+    int mid = (left + right) / 2;
+    auto node = new TreeNode(nums[mid]);
+    node->left = sorted_array_to_bst_recursive(nums, left, mid);
+    node->right = sorted_array_to_bst_recursive(nums, mid + 1, right);
+    return node;
+  }
+};
diff --git a/108/step2_3.cpp b/108/step2_3.cpp
new file mode 100644
index 0000000..7a324e6
--- /dev/null
+++ b/108/step2_3.cpp
@@ -0,0 +1,30 @@
+/*
+Time : O(N)
+Space : O(N)
+
+再帰を使わずに実装。
+https://github.com/colorbox/leetcode/pull/37
+を説いたときにある程度理解したつもりだったが、ポインタのポインタを使おうとするとだいぶ混乱する。
+
+*/
+class Solution {
+ public:
+  TreeNode* sortedArrayToBST(vector<int>& nums) {
+    stack<tuple<TreeNode**, int, int>> parents_and_ranges;
+    TreeNode *node;
+    parents_and_ranges.emplace(&node, 0, nums.size());
+    while (!parents_and_ranges.empty()){
+      auto [parent_node_pointer, left, right] = parents_and_ranges.top();
+      parents_and_ranges.pop();
+      if (left >= right) {
+        continue;
+      }
+      int mid = (left + right) / 2;
+      TreeNode* parent_node = new TreeNode(nums[mid]);
+      *parent_node_pointer = parent_node;
+      parents_and_ranges.emplace(&parent_node->left, left, mid);
+      parents_and_ranges.emplace(&parent_node->right, mid + 1, right);
+    }
+    return node;
+  }
+};
diff --git a/108/step3.cpp b/108/step3.cpp
new file mode 100644
index 0000000..b72e296
--- /dev/null
+++ b/108/step3.cpp
@@ -0,0 +1,24 @@
+/*
+再帰は慣れているので、ポインタに慣れるためにポインタを使用した解法でstep3
+*/
+class Solution {
+ public:
+  TreeNode* sortedArrayToBST(vector<int>& nums) {
+    stack<tuple<TreeNode**, int, int>> node_pointers_and_ranges;
+    TreeNode* root;
+    node_pointers_and_ranges.emplace(&root, 0, nums.size());
+    while (!node_pointers_and_ranges.empty()) {
+      auto [node_pointer, left, right] = node_pointers_and_ranges.top();
+      node_pointers_and_ranges.pop();
+      if (left >= right) {
+        continue;
+      }
+      int mid = (left + right) / 2;
+      TreeNode* node = new TreeNode(nums[mid]);
+      *node_pointer = node;
+      node_pointers_and_ranges.emplace(&node->left, left, mid);
+      node_pointers_and_ranges.emplace(&node->right, mid + 1, right);
+    }
+    return root;
+  }
+};