剑指Offer（四）--重建二叉树

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public TreeNode reConstructBinaryTree(int[] pre, int[] in) {
        if (pre == null || pre.length == 0 || in == null || in.length == 0) {
            return null;
        }
        TreeNode root = constructBinaryTree(pre, 0, pre.length - 1, in, 0, in.length-1);
        return root;
    }

    TreeNode constructBinaryTree(int[] pre, int startPre, int endPre, int[] in, int startIn, int endIn) {
        // 不符合条件直接返回null
        if (startPre > endPre || startIn > endIn) {
            return null;
        }
        // 构建根节点
        TreeNode root = new TreeNode(pre[startPre]);
        for (int index = startIn; index <= endIn; index++) {
            if (in[index] == pre[startPre]) {
                // 左子树
                root.left = constructBinaryTree(pre, startPre + 1, startPre + (index - startIn), in, startIn, index - 1);
                // 右子树
                root.right = constructBinaryTree(pre, (index - startIn) + startPre + 1, endPre, in, index + 1, endIn);
                break;
            }
        }
        return root;
    }
}