Input an integer array to determine whether the array is the result of a post-order traversal of a binary search tree. Returns true if yes, false otherwise. Suppose any two numbers of the input array are different from each other.

## code

static boolean isLastOrder(int[] lastOrder, int start, int end) { if (lastOrder == null || start < 0 || end < 0 || start > end) { return false; } int length = lastOrder.length; if (start >= length || end >= length) { return false; } // Postorder traversal of binary tree: left->right->root, so the last element of the array is the root of the binary tree int root = lastOrder[end]; // The characteristic of searching a binary tree is that any node in the left subtree is smaller than the root node, and any node in the right subtree is larger than the root node // Combined with post-order traversal, find the node position greater than the last element of the array, then the previous elements belong to the left subtree int rightStart = start; while (rightStart < end) { // Find the split point of the left and right subtrees, starting from the left and then the right subtree if (lastOrder[rightStart] > root){ break; } rightStart++; } int rightEnd = rightStart; while (rightEnd < end) { // If the right subtree has elements less than root, it means that it is not post-order traversal if (lastOrder[rightEnd] < root) { return false; } rightEnd++; } boolean leftIsLastOrder = true; // If the index of the starting element of the right subtree is greater than start, it means that the current root node has a left subtree if (rightStart > start) { leftIsLastOrder = isLastOrder(lastOrder, start, rightStart - 1); } boolean rightIsLastOrder = true; // If the index of the starting element of the right subtree is less than end - 1, it means that the current root node has a right subtree if (rightStart < end - 1) { rightIsLastOrder = isLastOrder(lastOrder, rightStart, end - 1); } // Both the left subtree and right subtree of the current root node satisfy postorder traversal return leftIsLastOrder && rightIsLastOrder; } public static void main(String[] args) { // 5 // 3 18 // 2 4 // Postorder traversal is 2, 4, 3, 18, 5 int[] order = {2, 4, 3, 18, 5}; System.out.print(isLastOrder(order, 0, order.length - 1)); }