105 从前序与中序遍历序列构造二叉树

import java.util.HashMap;
import java.util.Map;

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

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

public class Letcode105 {
    public TreeNode bulidTree(int[] preOrder, int[] inOrder) {
        int preLength = preOrder.length;
        int inLength = inOrder.length;

        // 用map存储中序遍历的数组及对应的下表
        Map<Integer, Integer> map = new HashMap<>();
        for (int i = 0; i < inLength; i++) {
            map.put(inOrder[i], i);
        }
        return bulidTree(preOrder, 0, preLength - 1,
                map, 0, inLength - 1);
    }

    private TreeNode bulidTree(int[] preOrder, int preLeft, int preRight, Map<Integer, Integer> map, int inLeft, int inRight) {
        // 不管是什么序，左边的边界一定小于右边的边界
        if (preLeft > preRight || inLeft > inRight) {
            return null;
        }

        // 前序遍历，第一个左边界一定是根节点
        int root = preOrder[preLeft];
        // 新建树
        TreeNode treeNode = new TreeNode(root);
        int pIndex = map.get(root);
        treeNode.left = bulidTree(preOrder, preLeft + 1, pIndex - inLeft + preLeft, map,
                inLeft, pIndex - 1);
        treeNode.right = bulidTree(preOrder,pIndex - inLeft + preLeft + 1, preRight,
                map,pIndex + 1,inRight);
        return treeNode;
    }
}