Build Binary Tree with Preorder and Inorder

BuildTreeWithPreorderInorder.swift

func buildTreeWithPreorder(preorder:[Int],andInorder  inorder:[Int]) -> TreeNode?{
    if preorder.isEmpty || inorder.isEmpty{
        return nil
    }
    
   
    var branchList = [TreeNode]()
    let root = TreeNode(val: 0)
    branchList.append(root)
    var preorderArray = [[Int]]()
    var inorderArray = [[Int]]()
    preorderArray.append(preorder)
    inorderArray.append(inorder)
    
    while (!preorderArray.isEmpty){
        var currentPreorder = preorderArray.removeAtIndex(0)
        var currentInorder = inorderArray.removeAtIndex(0)
        var currentBranch  = branchList.removeAtIndex(0)
        var branchNum = currentPreorder.removeAtIndex(0)
        currentBranch.val = branchNum
        var leftInorder = [Int]()
        var rightInorder = [Int]()
        var toLeft = true
        for num in currentInorder{
            if num == branchNum{
                toLeft = false
            }else{
                if toLeft{
                    leftInorder.append(num)
                }else{
                    rightInorder.append(num)
                }
            }
        }
        
        // 划分左右分支的前序节点出来
        var leftPreorder = [Int]()
        var rightPreorder = [Int]()
        
        for num in currentPreorder{
            if leftInorder.contains(num){
                leftPreorder.append(num)
            }else{
                rightPreorder.append(num)
            }
        }
        
        if !leftPreorder.isEmpty{
            let left = TreeNode(val: 0)
            currentBranch.left = left
            branchList.append(left)
            preorderArray.append(leftPreorder)
            inorderArray.append(leftInorder)
        }
        
        if !rightPreorder.isEmpty{
            let right = TreeNode(val: 0)
            currentBranch.right = right
            branchList.append(right)
            preorderArray.append(rightPreorder)
            inorderArray.append(rightInorder)
        }
    }
    
    return root
}