Merge pull request youngyangyang04#1164 from SianXiaoCHN/master

python3_0941.有效的山脉数组更新双指针方法_0225.用队列实现栈添加单队列优化方法_0112.路径总和精简递归回溯和递归返回条件，添加迭代法_0860.柠檬水找零 去除无用变量

Author: youngyangyang04

problems/0112.路径总和.md

@@ -496,25 +496,20 @@ class solution:
     def pathsum(self, root: treenode, targetsum: int) -> list[list[int]]:
 
         def traversal(cur_node, remain): 
-            if not cur_node.left and not cur_node.right and remain == 0: 
-                result.append(path[:])
-                return
-
-            if not cur_node.left and not cur_node.right: return 
+            if not cur_node.left and not cur_node.right:
+                if remain == 0: 
+                    result.append(path[:])
+            return
 
             if cur_node.left: 
                 path.append(cur_node.left.val)
-                remain -= cur_node.left.val
-                traversal(cur_node.left, remain)
+                traversal(cur_node.left, remain-cur_node.left.val)
                 path.pop()
-                remain += cur_node.left.val
 
             if cur_node.right: 
                 path.append(cur_node.right.val)
-                remain -= cur_node.right.val
-                traversal(cur_node.right, remain)
+                traversal(cur_node.right, remain-cur_node.left.val)
                 path.pop()
-                remain += cur_node.right.val
 
         result, path = [], []
         if not root: 
@@ -524,6 +519,30 @@ class solution:
         return result
 ```
 
+**迭代法，用第二个队列保存目前的总和与路径**
+```python
+class Solution:
+    def pathSum(self, root: Optional[TreeNode], targetSum: int) -> List[List[int]]:
+        if not root:
+            return []
+        que, temp = deque([root]), deque([(root.val, [root.val])])
+        result = []
+        while que:
+            for _ in range(len(que)):
+                node = que.popleft()
+                value, path = temp.popleft()
+                if (not node.left) and (not node.right):
+                    if value == targetSum:
+                        result.append(path)
+                if node.left:
+                    que.append(node.left)
+                    temp.append((node.left.val+value, path+[node.left.val]))
+                if node.right:
+                    que.append(node.right)
+                    temp.append((node.right.val+value, path+[node.right.val]))
+        return result
+```
+
 ## go
 
 112. 路径总和

problems/0225.用队列实现栈.md

@@ -354,6 +354,32 @@ class MyStack:
         return len(self.queue_in) == 0
 
 ```
+优化，使用一个队列实现
+```python
+class MyStack:
+
+    def __init__(self):
+        self.que = deque()
+
+    def push(self, x: int) -> None:
+        self.que.append(x)
+
+    def pop(self) -> int:
+        if self.empty():
+            return None
+        for i in range(len(self.que)-1):
+            self.que.append(self.que.popleft())
+        return self.que.popleft()
+
+    def top(self) -> int:
+        if self.empty():
+            return None
+        return self.que[-1]
+
+    def empty(self) -> bool:
+        return not self.que
+```
+
 
 Go：
 

problems/0455.分发饼干.md

@@ -32,7 +32,7 @@
 
 ## 思路
 
-为了了满足更多的小孩，就不要造成饼干尺寸的浪费。
+为了满足更多的小孩，就不要造成饼干尺寸的浪费。
 
 大尺寸的饼干既可以满足胃口大的孩子也可以满足胃口小的孩子，那么就应该优先满足胃口大的。
 

problems/0860.柠檬水找零.md

@@ -157,7 +157,7 @@ class Solution {
 ```python
 class Solution:
     def lemonadeChange(self, bills: List[int]) -> bool:
-        five, ten, twenty = 0, 0, 0
+        five, ten = 0, 0
         for bill in bills:
             if bill == 5:
                 five += 1
@@ -169,10 +169,8 @@ class Solution:
                 if ten > 0 and five > 0:
                     ten -= 1
                     five -= 1
-                    twenty += 1
                 elif five > 2:
                     five -= 3
-                    twenty += 1
                 else:
                     return False
         return True

problems/0941.有效的山脉数组.md

@@ -106,22 +106,15 @@ class Solution {
 ```python
 class Solution:
     def validMountainArray(self, arr: List[int]) -> bool:
-        if len(arr) < 3 :
-            return False
-
-        i = 1
-        flagIncrease = False # 上升
-        flagDecrease = False # 下降
-
-        while i < len(arr) and arr[i-1] < arr[i]:
-            flagIncrease = True
-            i += 1
-
-        while i < len(arr) and arr[i-1] > arr[i]:
-            flagDecrease = True
-            i += 1
-
-        return i == len(arr) and flagIncrease and flagDecrease
+        left, right = 0, len(arr)-1
+        
+        while left < len(arr)-1 and arr[left+1] > arr[left]:
+            left += 1
+        
+        while right > 0 and arr[right-1] > arr[right]:
+            right -= 1
+        
+        return left == right and right != 0 and left != len(arr)-1
 
 ```