u

2026-06-25 23:08:12 +10:00
parent 5c25a2e856
commit 416b0e9954
2 changed files with 116 additions and 0 deletions
@@ -0,0 +1,66 @@
+package top100liked
+
+import (
+	"fmt"
+	"testing"
+)
+
+// https://leetcode.cn/problems/path-sum-iii/description/?envType=study-plan-v2&envId=top-100-liked
+
+type TreeNode struct {
+	Val   int
+	Left  *TreeNode
+	Right *TreeNode
+}
+
+// 朴素解法，可以AC，但时间复杂度高
+// func pathSum(root *TreeNode, targetSum int) int {
+// 	if root == nil {
+// 		return 0
+// 	}
+// 	res := countFrom(root, targetSum)
+// 	res += pathSum(root.Left, targetSum)
+// 	res += pathSum(root.Right, targetSum)
+// 	return res
+// }
+// func countFrom(node *TreeNode, target int) int {
+// 	if node == nil {
+// 		return 0
+// 	}
+// 	c := 0
+// 	if node.Val == target {
+// 		c++
+// 	}
+// 	c += countFrom(node.Left, target-node.Val)
+// 	c += countFrom(node.Right, target-node.Val)
+// 	return c
+// }
+
+// 前缀和
+func pathSum(root *TreeNode, targetSum int) int {
+	m := make(map[int]int)
+	count := 0
+	m[0] = 1
+	var dfs func(*TreeNode, int)
+	dfs = func(root *TreeNode, cur int) {
+		if root == nil {
+			return
+		}
+		cur += root.Val
+		count += m[cur-targetSum]
+		m[cur]++
+		dfs(root.Left, cur)
+		dfs(root.Right, cur)
+		m[cur]--
+	}
+	dfs(root, 0)
+	return count
+}
+
+func Test(t *testing.T) {
+	input := &TreeNode{
+		Val:   1,
+		Right: &TreeNode{Val: 2, Right: &TreeNode{Val: 3, Right: &TreeNode{Val: 4, Right: &TreeNode{Val: 5}}}},
+	}
+	fmt.Println(pathSum(input, 3) == 2)
+}