剑指Offer-重构二叉树

题目描述：

输入某二叉树的前序遍历和中序遍历的结果，请重建出该二叉树。假设输入的前序遍历和中序遍历的结果中都不含重复的数字。例如输入前序遍历序列{1,2,4,7,3,5,6,8}和中序遍历序列{4,7,2,1,5,3,8,6}，则重建二叉树并返回。

public class Solution {
    class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;

        TreeNode(int x) {
            val = x;
        }
    }


    public TreeNode reConstructBinaryTree(int[] pre, int[] in) {
        if (pre != null && in != null)
            return solution(pre, in, 0, pre.length - 1, 0, in.length - 1);
        else
            return null;
    }

    private TreeNode solution(int[] pre, int[] in, int pStart, int pEnd, int iStart, int iEnd) {

        if (pStart > pEnd || iStart > iEnd)
            return null;

        TreeNode tree = new TreeNode(pre[pStart]);

        for (int root = iStart; root <= iEnd; root++)
            if (in[root] == pre[pStart]) {
                int leftLen = root - iStart;
                tree.left = solution(pre, in, pStart + 1, pStart + leftLen, iStart, root - 1);
                tree.right = solution(pre, in, pStart + leftLen + 1, pEnd, root + 1, iEnd);
                break;
            }
        return tree;
    }
}