Find Positive Integer Solution for a Given Equation

给一个匿名递增方程, 问所有小于z的答案的可能. 因为是递增方程,所以用二叉搜索.

/*
 * // This is the custom function interface.
 * // You should not implement it, or speculate about its implementation
 * class CustomFunction {
 *     // Returns f(x, y) for any given positive integers x and y.
 *     // Note that f(x, y) is increasing with respect to both x and y.
 *     // i.e. f(x, y) < f(x + 1, y), f(x, y) < f(x, y + 1)
 *     public int f(int x, int y);
 * };
 */
class Solution {
    public List<List<Integer>> findSolution(CustomFunction customfunction, int z) {
        List<List<Integer>> res = new ArrayList<>();
        int x = 1, y = z;
        while(x <= z && y > 0) {
                if(customfunction.f(x, y) == z) {
                        res.add(Arrays.asList(x++, y--));
                } else if(customfunction.f(x, y) > z) {
                        y--;
                } else {
                        x++;
                }
        }
        return res;
}
}