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public class Solution {
public TreeNode buildTree(int[] inorder, int[] postorder) { if (inorder == null || postorder == null) return null; if (inorder.length == 0 || postorder.length == 0) return null; if (inorder.length != postorder.length) return null;
TreeNode root = helper(inorder, 0, inorder.length - 1, postorder, 0, postorder.length - 1); return root; }
private TreeNode helper(int[] inorder, int instart, int inend, int[] postorder, int poststart, int postend) { if (instart > inend || poststart > postend) return null;
int root_val = postorder[postend]; TreeNode root = new TreeNode(root_val); int index = findIndex(inorder, instart, inend, root_val); root.left = helper(inorder, instart, index - 1, postorder, poststart, poststart + index - instart - 1); root.right = helper(inorder, index + 1, inend, postorder, poststart + index - instart, postend - 1); return root; }
private int findIndex(int[] nums, int start, int end, int target) { for (int i = start; i <= end; i++) { if (nums[i] == target) return i; } return -1; } }
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