Find Positive Integer Solution for a Given Equation LeetCode solution in JavaScript

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/**
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 * // This is the CustomFunction's API interface.
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 * // You should not implement it, or speculate about its implementation
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 * function CustomFunction() {
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 *     @param {integer, integer} x, y
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 *     @return {integer}
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 *     this.f = function(x, y) {
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 *         ...
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 *     };
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 * };
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 */
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/**
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 * @param {CustomFunction} customfunction
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 * @param {integer} z
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 * @return {integer[][]}
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 */
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var findSolution = function (customfunction, z) {
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  var ans = [];
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  var x = 1,
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    y = 1000;
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  while (x <= 1000 && y > 0) {
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    if (customfunction.f(x, y) > z) {
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      y--;
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    } else if (customfunction.f(x, y) < z) {
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      x++;
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    } else {
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      ans.push([x, y]);
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      x++;
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      y--;
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    }
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  }
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  return ans;
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};