剑指：重建二叉树

package demo;

import java.util.ArrayList;
import java.util.PriorityQueue;
import java.util.Stack;
import java.util.Comparator;

public class Solution {
    
    class TreeNode{
        int val;
        TreeNode left;
        TreeNode right;
        TreeNode(int x){
            val = x;
        }
    }
    
    public TreeNode buildTree(int[] preorder, int[] inorder){
        if (preorder == null || inorder == null || preorder.length == 0 || preorder.length != inorder.length) {
            return null;
        }
        return build(preorder, 0, preorder.length-1, inorder, 0, inorder.length-1);
    }
    
    private TreeNode build(int[] preorder, int startPre, int endPre, int[] inorder, int startIn, int endIn ){
        if(startPre>endPre || startIn>endIn) return null;
        
        TreeNode root = new TreeNode(preorder[startPre]);  //树根结点
        
        for(int i=startIn; i<=endIn; i++){
            if(inorder[i] == preorder[startPre]){
                //注意边界：i-startIn为偏移量
                root.left = build(preorder, startPre+1, startPre+i-startIn, inorder, startIn, i-1);
                root.right = build(preorder, i-startIn+startPre+1, endPre, inorder, i+1, endIn);
            }
        }
        return root;
    }
    
    public static void printPreOrder(TreeNode head) {
        if (head != null) {
            System.out.println(head.val);
            
            printPreOrder(head.left);
            printPreOrder(head.right);
        }
    }

    public static void main(String[] args) {
        int[] preorder =  {1,2,4,7,3,5,6,8};
        int[] inorder = {4,7,2,1,5,3,8,6};
        
        Solution rt = new Solution();
        Solution.TreeNode root = rt.buildTree(preorder, inorder);
        rt.printPreOrder(root);
    }
}