【剑指offer】 Java实现重建二叉树

GitHub上的代码链接

/**
 * @Author: DaleyZou
 * @Description: 重建二叉树
 * 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。
 * @Date: Created in 7:58 2019/1/29
 * @Modified By:
 */
public class ConstructBinaryTree_4 {

    public static class TreeNode {
      int val;
      TreeNode left;
      TreeNode right;
      TreeNode(int x) { val = x; }
    }

    /**
     * 缓存中序遍历数组每个值对应的索引
     */
    private Map<Integer, Integer> indexForInOrders = new HashMap<>();

    public TreeNode reConstructBinaryTree(int [] pre, int [] in) {
        for (int i = 0; i < in.length; i++){
            indexForInOrders.put(in[i], i);
        }

        return reConstructBinaryTree(pre, 0, pre.length - 1, 0);
}

    public TreeNode reConstructBinaryTree(int [] pre, int preL, int preR, int inL){
        if (preL > preR){
            return null;
        }

        TreeNode root = new TreeNode(pre[preL]);

        int inIndex = indexForInOrders.get(root.val);
        int leftTreeSize = inIndex - inL;

        root.left = reConstructBinaryTree(pre, preL + 1, preL + leftTreeSize, inL);
        root.right = reConstructBinaryTree(pre, preL + leftTreeSize + 1, preR, inL + leftTreeSize + 1);

        return root;
    }

    public static void main(String[] args){
        int[] pre = new int[]{1,2,4,7,3,5,6,8};
        int[] in = new int []{4,7,2,1,5,3,8,6};
        ConstructBinaryTree_4 constructBinaryTree = new ConstructBinaryTree_4();
        TreeNode treeNode = constructBinaryTree.reConstructBinaryTree(pre, in);
        System.out.println(treeNode.val);
    }
}