1,Construct Binary Tree from Inorder and Postorder Traversal
已知中序遍历序列,和后序遍历序列,通过这两个序列构造二叉树。
通过后序遍历序列的最后元素,我们总能找到根节点,然后通过这个值可以将中序序列分为两部分,然后在递归左右两部分。代码如下:
/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */ public class Solution { public TreeNode buildTree(int[] inorder, int[] postorder) { if(inorder == null || postorder == null || inorder.length == 0 || postorder.length == 0) return null; TreeNode root = new TreeNode(postorder[postorder.length-1]); int i = 0; for(i = 0; i < inorder.length; i++) { if(inorder[i] == postorder[postorder.length-1]) break; } int[] lp = Arrays.copyOfRange(postorder, 0, i); int[] li = Arrays.copyOfRange(inorder, 0, i); int[] rp = Arrays.copyOfRange(postorder, i, postorder.length - 1); int[] ri = Arrays.copyOfRange(inorder, i + 1, inorder.length); root.left = buildTree(li, lp); root.right = buildTree(ri, rp); return root; } }
2,Construct Binary Tree from Preorder and Inorder Traversal
已知中序遍历序列和前序遍历序列,构造二叉树
和上一题类似,我们从前序遍历序列中的第一位元素,来将中序序列分为两部分,即分别为根节点的左子树和右子树,递归左子树和右子树,代码如下:
/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */ public class Solution { public TreeNode buildTree(int[] preorder, int[] inorder) { if(preorder == null || inorder == null || preorder.length == 0) return null; TreeNode root = new TreeNode(preorder[0]); int i = 0; for(i = 0; i < inorder.length; i++) { if(inorder[i] == preorder[0]) break; } root.left = buildTree(Arrays.copyOfRange(preorder,1, i+1), Arrays.copyOfRange(inorder,0,i)); root.right = buildTree(Arrays.copyOfRange(preorder, i+1, preorder.length), Arrays.copyOfRange(inorder, i+1, inorder.length)); return root; } }