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Update 0606-construct-string-from-binary-tree.kt

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Original file line number	Diff line number	Diff line change
`@@ -1,3 +1,4 @@`
	`1`	`+// recursion`
`1`	`2`	`classSolution {`
`2`	`3`	`funtree2str(root:TreeNode?):String {`
`3`	`4`	`val res=StringBuilder()`
`@@ -22,3 +23,39 @@ class Solution {`
`22`	`23`	`return res.toString()`
`23`	`24`	`}`
`24`	`25`	`}`
	`26`	`+`
	`27`	`+// iterative`
	`28`	`+classSolution {`
	`29`	`+funtree2str(root:TreeNode?):String {`
	`30`	`+ root?:return""`
	`31`	`+`
	`32`	`+val res=StringBuilder()`
	`33`	`+val stack=LinkedList<TreeNode?>().apply { addLast(root) }`
	`34`	`+val visited=HashSet<TreeNode?>()`
	`35`	`+`
	`36`	`+while (stack.isNotEmpty()) {`
	`37`	`+val cur= stack.peekLast()`
	`38`	`+`
	`39`	`+if (curin visited) {`
	`40`	`+ stack.removeLast()`
	`41`	`+ res.append(")")`
	`42`	`+ }else {`
	`43`	`+ visited.add(cur)`
	`44`	`+ res.append("(")`
	`45`	+ res.append(cur?.`val`)
	`46`	`+`
	`47`	`+if (cur?.left==null&& cur?.right!=null)`
	`48`	`+ res.append("()")`
	`49`	`+`
	`50`	`+if (cur?.right!=null)`
	`51`	`+ stack.addLast(cur?.right)`
	`52`	`+if (cur?.left!=null)`
	`53`	`+ stack.addLast(cur?.left)`
	`54`	`+ }`
	`55`	`+ }`
	`56`	`+`
	`57`	`+ res.deleteCharAt(0)`
	`58`	`+ res.deleteCharAt(res.lastIndex)`
	`59`	`+return res.toString()`
	`60`	`+ }`
	`61`	`+}`

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