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feat: add swift implementation to lcof2 problem: No.050 (doocs#3065)

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Original file line number	Diff line number	Diff line change
`@@ -241,6 +241,57 @@ function pathSum(root: TreeNode \| null, targetSum: number): number {`
`241`	`241`	`}`
`242`	`242`	```
`243`	`243`
	`244`	`+#### Swift`
	`245`	`+`
	`246`	+```swift
	`247`	`+/* class TreeNode {`
	`248`	`+* var val: Int`
	`249`	`+* var left: TreeNode?`
	`250`	`+* var right: TreeNode?`
	`251`	`+* init() {`
	`252`	`+* self.val = 0`
	`253`	`+* self.left = nil`
	`254`	`+* self.right = nil`
	`255`	`+* }`
	`256`	`+* init(_ val: Int) {`
	`257`	`+* self.val = val`
	`258`	`+* self.left = nil`
	`259`	`+* self.right = nil`
	`260`	`+* }`
	`261`	`+* init(_ val: Int, _ left: TreeNode?, _ right: TreeNode?) {`
	`262`	`+* self.val = val`
	`263`	`+* self.left = left`
	`264`	`+* self.right = right`
	`265`	`+* }`
	`266`	`+* }`
	`267`	`+*/`
	`268`	`+`
	`269`	`+class Solution {`
	`270`	`+ private var cnt = [Int: Int]()`
	`271`	`+ private var targetSum: Int = 0`
	`272`	`+`
	`273`	`+ func pathSum(_ root: TreeNode?, _ targetSum: Int) -> Int {`
	`274`	`+ self.targetSum = targetSum`
	`275`	`+ cnt[0] = 1`
	`276`	`+ return dfs(root, 0)`
	`277`	`+ }`
	`278`	`+`
	`279`	`+ private func dfs(_ node: TreeNode?, _ s: Int) -> Int {`
	`280`	`+ guard let node = node else {`
	`281`	`+ return 0`
	`282`	`+ }`
	`283`	`+ var s = s`
	`284`	`+ s += node.val`
	`285`	`+ var ans = cnt[s - targetSum, default: 0]`
	`286`	`+ cnt[s, default: 0] += 1`
	`287`	`+ ans += dfs(node.left, s)`
	`288`	`+ ans += dfs(node.right, s)`
	`289`	`+ cnt[s]! -= 1`
	`290`	`+ return ans`
	`291`	`+ }`
	`292`	`+}`
	`293`	+```
	`294`	`+`
`244`	`295`	`<!-- tabs:end -->`
`245`	`296`
`246`	`297`	`<!-- solution:end -->`

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Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,46 @@`
	`1`	`+/* class TreeNode {`
	`2`	`+* var val: Int`
	`3`	`+* var left: TreeNode?`
	`4`	`+* var right: TreeNode?`
	`5`	`+* init() {`
	`6`	`+* self.val = 0`
	`7`	`+* self.left = nil`
	`8`	`+* self.right = nil`
	`9`	`+* }`
	`10`	`+* init(_ val: Int) {`
	`11`	`+* self.val = val`
	`12`	`+* self.left = nil`
	`13`	`+* self.right = nil`
	`14`	`+* }`
	`15`	`+* init(_ val: Int, _ left: TreeNode?, _ right: TreeNode?) {`
	`16`	`+* self.val = val`
	`17`	`+* self.left = left`
	`18`	`+* self.right = right`
	`19`	`+* }`
	`20`	`+* }`
	`21`	`+*/`
	`22`	`+`
	`23`	`+class Solution {`
	`24`	`+ private var cnt = [Int: Int]()`
	`25`	`+ private var targetSum: Int = 0`
	`26`	`+`
	`27`	`+ func pathSum(_ root: TreeNode?, _ targetSum: Int) -> Int {`
	`28`	`+ self.targetSum = targetSum`
	`29`	`+ cnt[0] = 1`
	`30`	`+ return dfs(root, 0)`
	`31`	`+ }`
	`32`	`+`
	`33`	`+ private func dfs(_ node: TreeNode?, _ s: Int) -> Int {`
	`34`	`+ guard let node = node else {`
	`35`	`+ return 0`
	`36`	`+ }`
	`37`	`+ var s = s`
	`38`	`+ s += node.val`
	`39`	`+ var ans = cnt[s - targetSum, default: 0]`
	`40`	`+ cnt[s, default: 0] += 1`
	`41`	`+ ans += dfs(node.left, s)`
	`42`	`+ ans += dfs(node.right, s)`
	`43`	`+ cnt[s]! -= 1`
	`44`	`+ return ans`
	`45`	`+ }`
	`46`	`+}`

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