题解 | #从中序与后序遍历序列构造二叉树#

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 inorder int整型一维数组 中序遍历序列
     * @param postorder int整型一维数组 后序遍历序列
     * @return TreeNode类
     */
    public TreeNode buildTree (int[] inorder, int[] postorder) {
        // write code here
        int n = inorder.length;
        return buildingTree(inorder, postorder, 0, n - 1, 0, n - 1);
    }
    private TreeNode buildingTree(int[] inorder, int[] postorder, int inStart,
                                  int inEnd, int postStart, int postEnd) {
        int rootIndex = inStart;
        TreeNode root = new TreeNode(postorder[postEnd]);
        for (int i = inStart; i <= inEnd; i++) {
            if (postorder[postEnd] == inorder[i]) {
                rootIndex = i;
                break;
            }
        }
        int offset = rootIndex - inStart;
        if (rootIndex > inStart) {//存在左子树
            root.left = buildingTree(inorder, postorder, inStart, rootIndex - 1, postStart,
                                     postStart + offset - 1);
        }
        if (rootIndex < inEnd) {//存在右子树
            root.right = buildingTree(inorder, postorder, rootIndex + 1, inEnd,
                                      postStart + offset, postEnd - 1);

        }
        return root;
    }
}