LeetCode 106. Construct Binary Tree from Inorder and Postorder Traversal (用中序和后序树遍历来建立二叉树)
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Given inorder and postorder traversal of a tree, construct the binary tree.
Note:
You may assume that duplicates do not exist in the tree.
题目标签:Array, Tree
这到题目和105 几乎是一摸一样的,唯一的区别就是把pre-order 换成 post-order。因为post-order是最后一个数字是root,所以要从右向左的遍历。还需要把helper function 里的 right child 和 left child 顺序更换一下,并且要把相关的代入值也改一下。具体可以看code。
Java Solution:
Runtime beats 68.55%
完成日期:08/26/2017
关键词:Array, Tree
关键点:递归;利用post-order 和 in-order 的位置关系递归
1 /** 2 * Definition for a binary tree node. 3 * public class TreeNode { 4 * int val; 5 * TreeNode left; 6 * TreeNode right; 7 * TreeNode(int x) { val = x; } 8 * } 9 */ 10 class Solution 11 { 12 public TreeNode buildTree(int[] inorder, int[] postorder) 13 { 14 Map<Integer, Integer> inMap = new HashMap<Integer, Integer>(); 15 16 // save inorder number as key, position as value into map 17 for(int i=0; i<inorder.length; i++) 18 inMap.put(inorder[i], i); 19 20 21 TreeNode root = helper(postorder, postorder.length-1, 0, inorder.length - 1, inMap); 22 23 return root; 24 } 25 26 public TreeNode helper(int[] postorder, int postEnd, int inStart, int inEnd, 27 Map<Integer, Integer> inMap) 28 { 29 if(inStart > inEnd) 30 return null; 31 32 int rootVal = postorder[postEnd]; 33 TreeNode root = new TreeNode(rootVal); 34 int inRoot = inMap.get(rootVal); // position in inOrder 35 36 /* inStart & inEnd: for left child, move inEnd to the left of root 37 * for right child, move inStart to the right of root */ 38 root.right = helper(postorder, postEnd - 1, inRoot + 1, inEnd, inMap); 39 /* postStart: for right left, go to inorder to check how many right children does root have, 40 * add it into postorder to skip them to reach left child */ 41 root.left = helper(postorder, postEnd - (inEnd - inRoot) - 1, inStart, inRoot - 1, inMap); 42 43 return root; 44 } 45 }
参考资料:N/A
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