牛客 重建二叉树

Posted 2020-11-02 kaibing

//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 test_4 {

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

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

    public static TreeNode reConstructBinaryTree(int[] pre, int[] in) {
        TreeNode tree = reConstructBinaryTree(pre, 0, pre.length - 1, in, 0, in.length - 1);
        return tree;
    }

    public static TreeNode reConstructBinaryTree(int[] pre, int preStartIndex, int preEndIndex, int[] in, int inStartIndex, int inEndIndex) {
        if (preStartIndex > preEndIndex || inStartIndex > inEndIndex) {
            return null;
        }
        TreeNode tree = new TreeNode(pre[preStartIndex]);

        for (int i = inStartIndex; i <= inEndIndex; i++) {
            if (pre[preStartIndex] == in[i]) {
                tree.left = reConstructBinaryTree(pre, preStartIndex + 1, preStartIndex + i - inStartIndex, in, inStartIndex, i - 1);
                tree.right = reConstructBinaryTree(pre, preStartIndex + i - inStartIndex + 1, preEndIndex, in, i + 1, inEndIndex);
            }
        }
        return tree;
    }

    public static void main(String[] args) {
        int preOrder[] = {1, 2, 4, 7, 3, 5, 6, 8};
        int inOrder[] = {4, 7, 2, 1, 5, 3, 8, 6};
        reConstructBinaryTree(preOrder, inOrder);

    }
}