Leetcode——106. 从中序与后序遍历序列构造二叉树

106. 从中序与后序遍历序列构造二叉树

根据一棵树的中序遍历与后序遍历构造二叉树。

注意:
你可以假设树中没有重复的元素。

例如，给出

中序遍历 inorder = [9,3,15,20,7]
后序遍历 postorder = [9,15,7,20,3]
返回如下的二叉树：

  3
 / \
9  20
  /  \
 15   7

代码：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode buildTree(int[] inorder, int[] postorder) {
        if(null == inorder || postorder == null || inorder.length != postorder.length || inorder.length <= 0) 
            return null;
        return createTree(inorder, 0, inorder.length-1, postorder, 0, postorder.length-1);
    }

    public TreeNode createTree(int[] inorder, int is, int ie, int[] postorder, int ps, int pe) {
        if(is > ie || ps > pe)
            return null;
        TreeNode root = new TreeNode(postorder[pe]);
        int i = searchArray(inorder, postorder[pe]);
        root.left = createTree(inorder, is, i - 1, postorder, ps, ps + i - is - 1);
        root.right = createTree(inorder, i + 1, ie, postorder, ps + i - is, pe - 1);
        return root;
    }

    public int searchArray(int[] nums, int key) {   //key一定在nums内
        int i = 0;
        while(nums[i++] != key);
        return --i;
    }
}