1. var Solution = function(nums) {
    2.   this.nums = nums;
    3. };
    5. Solution.prototype.pick = function(target) {
    6.   let ans = 0;
    7.   for (let i = 0, cnt = 0; i < this.nums.length; ++i) {
    8.     if (this.nums[i] == target) {
    9.       ++cnt; // 第 cnt 次遇到 target
    10.       if (Math.floor(Math.random() * cnt) === 0) {
    11.         ans = i;
    12.       }
    13.     }
    14.   }
    15.   return ans;
    16. };

    解决大文件随机抽取同一值的数据