[printjin-gmailcom] week 11 solutions

binary-tree-maximum-path-sum/printjin-gmailcom.py

@@ -0,0 +1,12 @@
+class Solution:
+    def maxPathSum(self, root):
+        self.max_sum = float('-inf')
+        def dfs(node):
+            if not node:
+                return 0
+            left = max(dfs(node.left), 0)
+            right = max(dfs(node.right), 0)
+            self.max_sum = max(self.max_sum, node.val + left + right)
+            return node.val + max(left, right)
+        dfs(root)
+        return self.max_sum

graph-valid-tree/printjin-gmailcom.py

@@ -0,0 +1,16 @@
+class Solution:
+    def valid_tree(self, n, edges):
+        if len(edges) != n - 1:
+            return False
+        parent = [i for i in range(n)]
+        def find(x):
+            while parent[x] != x:
+                parent[x] = parent[parent[x]]
+                x = parent[x]
+            return x
+        for u, v in edges:
+            pu, pv = find(u), find(v)
+            if pu == pv:
+                return False
+            parent[pu] = pv
+        return True

merge-intervals/printjin-gmailcom.py

@@ -0,0 +1,10 @@
+class Solution:
+    def merge(self, intervals):
+        intervals.sort(key=lambda x: x[0])
+        merged = []
+        for interval in intervals:
+            if not merged or merged[-1][1] < interval[0]:
+                merged.append(interval)
+            else:
+                merged[-1][1] = max(merged[-1][1], interval[1])
+        return merged

missing-number/printjin-gmailcom.py

@@ -0,0 +1,6 @@
+class Solution:
+    def missingNumber(self, nums):
+        n = len(nums)
+        expected_sum = n * (n + 1) // 2
+        actual_sum = sum(nums)
+        return expected_sum - actual_sum

reorder-list/printjin-gmailcom.py

@@ -0,0 +1,18 @@
+class Solution:
+    def reorderList(self, head):
+        if not head:
+            return
+        nodes = []
+        current = head
+        while current:
+            nodes.append(current)
+            current = current.next
+        i, j = 0, len(nodes) - 1
+        while i < j:
+            nodes[i].next = nodes[j]
+            i += 1
+            if i == j:
+                break
+            nodes[j].next = nodes[i]
+            j -= 1
+        nodes[i].next = None