题解 | 重建二叉树

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param preOrder int整型一维数组 
     * @param vinOrder int整型一维数组 
     * @return TreeNode类
     */
    private Map<Integer, Integer> inToIdx;
    public TreeNode reConstructBinaryTree (int[] preOrder, int[] vinOrder) {
        inToIdx = new HashMap<>();
        int n = preOrder.length;
        for (int i=0; i<n; i++) {
            inToIdx.put(vinOrder[i], i);
        }
        return build(preOrder, vinOrder, 0, n-1, 0, n-1);
    }

    private TreeNode build(int []pre, int[] in, int pl, int pr, int il, int ir) {
        if (pr < pl || ir < il) return null;
        TreeNode node = new TreeNode(pre[pl]);
        int k = inToIdx.get(pre[pl]);
        node.left = build(pre, in, pl+1, pl+k-il, il, k-1);
        node.right = build(pre, in, pl+k-il+1, pr, k+1, ir);
        return node;
    }
}