Binary Search Tree Lookup in Java

// Binary Search Tree Lookup in Java
//
// PURPOSE
// A binary search tree chooses a branch by comparing the target with the current node key.
//
// METHOD
// At each node return on equality, otherwise follow the left child for a smaller target or
// the right child for a larger target.
//
// COMPLEXITY
// O(height) time and O(1) auxiliary space; height is O(log n) only when the tree is
// balanced.
//
// CONTRACT
// keys,left_child,right_child have n entries; -1 means absent. Must be a valid BST. Returns
// node ID or -1.
// Public APIs infer array lengths except in C, which accepts pointer/length pairs. Vertex
// counts and matrix dimensions remain explicit. Arrays must contain signed 32-bit integers.
// Sorted order, correct tree topology, and caller-owned data-structure invariants are
// preconditions; checking them on every query would change query complexity.
// Scalar and shape checks are included. Value-range checks are included where needed to
// prevent invalid indexing or nontermination. Allocation sizes are capped at 4,000,000
// workspace entries. Arithmetic outside Python raises an error on 32-bit overflow; Python
// uses arbitrary precision. Native allocation failures propagate. In-place outputs may be
// partly updated after arithmetic failure. These are single-threaded, in-memory
// implementations.
//
// LANGUAGE DETAILS
// Java 17+: primitive int arrays, Arrays.binarySearch for LIS, and Math exact arithmetic.
// Invalid input throws IllegalArgumentException; arithmetic overflow throws
// ArithmeticException.
//
// EXAMPLE
// Expected return: 2.
// Run: javac AlgorithmExample.java && java AlgorithmExample
// Tests for this exact source artifact run before publication. The accompanying source
// archive includes unit cases, runtime results, and source hashes.

class AlgorithmExample {
    static void alg_require(boolean valid) { if (!valid) throw new IllegalArgumentException("invalid algorithm input"); }
    static int alg_size(int n) { alg_require(n>=0 && n<=4000000); return n; }

    static int bst_lookup(int[] keys, int[] left_child, int[] right_child, int root, int target) {
        java.util.Objects.requireNonNull(keys, "keys");
        java.util.Objects.requireNonNull(left_child, "left_child");
        java.util.Objects.requireNonNull(right_child, "right_child");
        int n = keys.length;
        alg_require(((n >= 0) && (n <= 1000000)));
        alg_require((keys.length == n));
        alg_require((left_child.length == n));
        alg_require((right_child.length == n));
        alg_require(((root >= Math.negateExact(1)) && (root < n)));
        for (int alg_i = 0; alg_i < left_child.length; alg_i++) {
            int alg_value = left_child[alg_i];
            alg_require(((alg_value >= Math.negateExact(1)) && (alg_value < n)));
        }
        for (int alg_i = 0; alg_i < right_child.length; alg_i++) {
            int alg_value = right_child[alg_i];
            alg_require(((alg_value >= Math.negateExact(1)) && (alg_value < n)));
        }
        while ((root != Math.negateExact(1))) {
            if ((keys[root] == target)) {
                return root;
            }
            if ((target < keys[root])) {
                root = left_child[root];
            } else {
                root = right_child[root];
            }
        }
        return Math.negateExact(1);
    }

    public static void main(String[] args) {
        int[] keys = new int[] {4, 2, 6};
        int[] left_child = new int[] {1, -1, -1};
        int[] right_child = new int[] {2, -1, -1};
        int root = 0;
        int target = 6;
        int result = bst_lookup(keys, left_child, right_child, root, target);
        System.out.println(result);
    }
}