From 19b7bd0cb4599110fb30d18192c22c602b154f64 Mon Sep 17 00:00:00 2001
From: Hiro Funatsuka <hiro.funatsuka@gmail.com>
Date: Mon, 27 Apr 2026 23:37:59 +0900
Subject: [PATCH] Solve path sum

---
 README.md |  2 +-
 step1.md  | 29 ++++++++++++++++++++++++++--
 step2.md  | 56 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 step3.md  | 10 +++++++---
 4 files changed, 91 insertions(+), 6 deletions(-)

diff --git a/README.md b/README.md
index 0ee68b0..24302a2 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,6 @@
 # LeetCode
 
-## 問題: [Link Text](URL)
+## 問題: [112. Path Sum](https://leetcode.com/problems/path-sum/description/)
 
 ## 前提
 
diff --git a/step1.md b/step1.md
index 54ebbc1..9c2462e 100644
--- a/step1.md
+++ b/step1.md
@@ -1,7 +1,32 @@
 # Step 1
 
+- recursionで試したが、上手くいかなかったのでまずiterativeで試す
+
 ```python
+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+class Solution:
+    def hasPathSum(self, root: Optional[TreeNode], targetSum: int) -> bool:
+        if root is None:
+            return False
+
+        stack = [(root, root.val)]
+        while stack:
+            node, current_sum = stack.pop()
+            if node:
+                if node.left is None and node.right is None and current_sum == targetSum:
+                    return True
+                if node.left is not None:
+                    stack.append((node.left, current_sum + node.left.val))
+                if node.right is not None:
+                    stack.append((node.right, current_sum + node.right.val))
+        return False
 ```
-時間計算量:
 
-空間計算量:
+時間計算量: O(n)
+
+空間計算量: O(n)
diff --git a/step2.md b/step2.md
index 6ec3360..2996861 100644
--- a/step2.md
+++ b/step2.md
@@ -1,4 +1,60 @@
 # Step 2
 
+## Iterative DFS
+
+```python
+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+class Solution:
+    def hasPathSum(self, root: Optional[TreeNode], targetSum: int) -> bool:
+        if root is None:
+            return False
+
+        nodes_and_sums = [(root, root.val)]
+        while nodes_and_sums:
+            node, current_sum = nodes_and_sums.pop()
+            if node:
+                if node.left is None and node.right is None and current_sum == targetSum:
+                    return True
+                if node.left is not None:
+                    nodes_and_sums.append((node.left, current_sum + node.left.val))
+                if node.right is not None:
+                    nodes_and_sums.append((node.right, current_sum + node.right.val))
+        return False
+```
+
+時間計算量: O(n)
+
+空間計算量: O(n)
+
+## Recursive DFS
+
+- 他の人のコードを拝見
+
 ```python
+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+class Solution:
+    def hasPathSum(self, root: Optional[TreeNode], targetSum: int) -> bool:
+        def dfs(node, current_sum):
+            if node is None:
+                return False
+
+            current_sum += node.val
+            if node.left is None and node.right is None:
+                return current_sum == targetSum
+            return dfs(node.left, current_sum) or dfs(node.right, current_sum)
+        return dfs(root, 0)
 ```
+
+時間計算量: O(n)
+
+空間計算量: O(n)
diff --git a/step3.md b/step3.md
index 522daa6..87a6c88 100644
--- a/step3.md
+++ b/step3.md
@@ -1,9 +1,13 @@
 # Step 3
 
+- 思いつかなかったRecursive DFSで練習
+
 ```python
+
 ```
-1回目: 分　秒
 
-2回目: 分　秒
+1回目: 1分　24秒
+
+2回目: 1分　30秒
 
-3回目: 分　秒
+3回目: 1分　18秒