|
| 1 | +/** |
| 2 | + * Definition for a binary tree node. |
| 3 | + * public class TreeNode { |
| 4 | + * int val; |
| 5 | + * TreeNode left; |
| 6 | + * TreeNode right; |
| 7 | + * TreeNode() {} |
| 8 | + * TreeNode(int val) { this.val = val; } |
| 9 | + * TreeNode(int val, TreeNode left, TreeNode right) { |
| 10 | + * this.val = val; |
| 11 | + * this.left = left; |
| 12 | + * this.right = right; |
| 13 | + * } |
| 14 | + * } |
| 15 | + */ |
| 16 | +classSolution { |
| 17 | +publicTreeNodeconstructFromPrePost(int[]preorder,int[]postorder) { |
| 18 | +intn =preorder.length; |
| 19 | +Map<Integer,Integer>postorderIndex =newHashMap<>(); |
| 20 | +for (inti =0;i <n;i++) { |
| 21 | +postorderIndex.put(postorder[i],i); |
| 22 | + } |
| 23 | +returnconstructTree(0,n -1,0,preorder,postorderIndex); |
| 24 | + } |
| 25 | + |
| 26 | +privateTreeNodeconstructTree(intpreStart,intpreEnd,intpostStart,int[]preorder,Map<Integer,Integer>postorderIndex) { |
| 27 | +if (preStart >preEnd) { |
| 28 | +returnnull; |
| 29 | + } |
| 30 | +if (preStart ==preEnd) { |
| 31 | +returnnewTreeNode(preorder[preStart]); |
| 32 | + } |
| 33 | +intleftRoot =preorder[preStart +1]; |
| 34 | +intleftCount =postorderIndex.get(leftRoot) -postStart +1; |
| 35 | +TreeNoderoot =newTreeNode(preorder[preStart]); |
| 36 | +root.left =constructTree(preStart +1,preStart +leftCount,postStart,preorder,postorderIndex); |
| 37 | +root.right =constructTree(preStart +leftCount +1,preEnd,postStart +leftCount,preorder,postorderIndex); |
| 38 | +returnroot; |
| 39 | + } |
| 40 | +} |