java实现——006重建二叉树

public class T006 {
    public static void main(String[] args){
        int pre[] = {1,2,4,7,3,5,6,8};
        int in[] = {4,7,2,1,5,3,8,6};
        preorderTraversalRec(construct(pre,in,8));

    }
    public static void preorderTraversalRec(TreeNode root) {
        if (root == null) {
            return;
        }
        System.out.print(root.val + " ");
        preorderTraversalRec(root.left);
        preorderTraversalRec(root.right);
    }
    public static TreeNode construct(int[] pre,int[] in,int length){
        if(pre == null ||in == null||length<=0)
            return null;
        return constructCore(pre,in,0,length-1,0,length-1);
        
    }
    public static TreeNode constructCore(int[] pre,int[] in,int startPre,int endPre,int startIn,int endIn){
        int rootValue = pre[startPre];
        TreeNode root = new TreeNode(rootValue);
        root.left = null;
        root.right = null;
        if(startPre == endPre){
            if(startIn == endIn&&startPre==startIn){
                System.out.println("root");
                return root;
            }
        }    
        int rootIn = startIn;
        while(rootIn<=endIn&&in[rootIn]!=rootValue){
            ++ rootIn;
        }
        if(rootIn == endIn&&in[rootIn]!=rootValue){
            System.out.println("Invalid input2");
        }
        int leftLength = rootIn - startIn;
        int leftPreEnd = startPre + leftLength;
        if(leftLength>0){
            root.left = constructCore(pre,in,startPre+1,leftPreEnd,startIn,rootIn-1);
        }
        if(leftLength<endPre-startPre){
            root.right = constructCore(pre,in,leftPreEnd+1,endPre,rootIn+1,endIn);
        }
        return root;
        
    }
    private static class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;

        public TreeNode(int val) {
            this.val = val;
        }
    }
}