Merge pull request #559 from 0xff-dev/331

Add solution and test-cases for problem 331

leetcode/301-400/0331.Verify-Preorder-Serialization-of-a-Binary-Tree/README.md

@@ -1,28 +1,42 @@
 # [331.Verify Preorder Serialization of a Binary Tree][title]
 
-> [!WARNING|style:flat]
-> This question is temporarily unanswered if you have good ideas. Welcome to [Create Pull Request PR](https://github.com/kylesliu/awesome-golang-algorithm)
-
 ## Description
+One way to serialize a binary tree is to use **preorder traversal**. When we encounter a non-null node, we record the node's value. If it is a null node, we record using a sentinel value such as `'#'`.
+
+![example](./pre-tree.jpg)
+
+For example, the above binary tree can be serialized to the string `"9,3,4,#,#,1,#,#,2,#,6,#,#"`, where `'#'` represents a null node.
+
+Given a string of comma-separated values `preorder`, return `true` if it is a correct preorder traversal serialization of a binary tree.
+
+It is **guaranteed** that each comma-separated value in the string must be either an integer or a character `'#'` representing null pointer.
+
+You may assume that the input format is always valid.
+
+- For example, it could never contain two consecutive commas, such as `"1,,3"`.
+
+**Note**: You are not allowed to reconstruct the tree.
 
 **Example 1:**
 
 ```
-Input: a = "11", b = "1"
-Output: "100"
+Input: preorder = "9,3,4,#,#,1,#,#,2,#,6,#,#"
+Output: true
 ```
 
-## 题意
-> ...
-
-## 题解
+**Example 2:**
 
-### 思路1
-> ...
-Verify Preorder Serialization of a Binary Tree
-```go
+```
+Input: preorder = "1,#"
+Output: false
 ```
 
+**Example 3:**
+
+```
+Input: preorder = "9,#,#,1"
+Output: false
+```
 
 ## 结语
 

leetcode/301-400/0331.Verify-Preorder-Serialization-of-a-Binary-Tree/Solution.go

@@ -1,5 +1,55 @@
 package Solution
 
-func Solution(x bool) bool {
-	return x
+import "strings"
+
+type pair331 struct {
+	sharp, index int
+}
+
+func Solution(preorder string) bool {
+	if preorder == "#" {
+		return true
+	}
+	items := strings.Split(preorder, ",")
+	l := len(items)
+	if l < 3 {
+		return false
+	}
+	pairs := make([]pair331, 0)
+	for i := 0; i < l; i++ {
+		if items[i] != "#" {
+			pairs = append(pairs, pair331{index: i})
+			continue
+		}
+		last := len(pairs) - 1
+		if last == -1 {
+			return false
+		}
+		if items[pairs[last].index] == "#" || pairs[last].sharp >= 2 {
+			pairs = append(pairs, pair331{index: i})
+			continue
+		}
+		pairs[last].sharp++
+	}
+
+	stack := []int{-1, -1}
+	for _, pair := range pairs {
+		if pair.sharp == 1 {
+			stack = append(stack, pair.index)
+			continue
+		}
+		if pair.sharp == 0 && items[pair.index] != "#" {
+			stack = append(stack, pair.index, pair.index)
+			continue
+		}
+		ll := len(stack)
+		if ll == 0 {
+			break
+		}
+		for ll >= 2 && stack[ll-1] != stack[ll-2] {
+			ll--
+		}
+		stack = stack[:ll-1]
+	}
+	return len(stack) == 1
 }

leetcode/301-400/0331.Verify-Preorder-Serialization-of-a-Binary-Tree/Solution_test.go

@@ -10,12 +10,17 @@ func TestSolution(t *testing.T) {
 	//	测试用例
 	cases := []struct {
 		name   string
-		inputs bool
+		inputs string
 		expect bool
 	}{
-		{"TestCase", true, true},
-		{"TestCase", true, true},
-		{"TestCase", false, false},
+		{"TestCase1", "9,3,4,#,#,1,#,#,2,#,6,#,#", true},
+		{"TestCase2", "1,#", false},
+		{"TestCase3", "9,3,#,1", false},
+		{"TestCase4", "9,#,93,#,9,9,#,#,#", true},
+		{"TestCase5", "9,9,91,#,#,9,#,49,#,#,#", true},
+		{"TestCase6", "8,#,5,#,2,5,#,7,9,#,8,#,#,#,#", true},
+		{"TestCase7", "1,#,#,#,#", false},
+		{"TestCase8", "1,#,#", true},
 	}
 
 	//	开始测试
@@ -30,10 +35,10 @@ func TestSolution(t *testing.T) {
 	}
 }
 
-//	压力测试
+// 压力测试
 func BenchmarkSolution(b *testing.B) {
 }
 
-//	使用案列
+// 使用案列
 func ExampleSolution() {
 }