力扣——最接近的三数之和

输入：
nums = [-1,2,1,-4], target = 1
输出：
2
解释：
与 target 最接近的和是 2 (-1 + 2 + 1 = 2) 

let threeSumClosest = function (nums, target) {
            nums.sort((a, b) => a - b)
            //先令差值为最大值
            let dif = Number.MAX_VALUE
            for (let i = 0; i < nums.length; i++) {
                let n1 = nums[i]
                let left = i + 1, right = nums.length - 1
       
                while (left < right) {
                    let n2 = nums[left] , n3 = nums[right]
                    let num = n1 + n2 + n3
                    // 比较那个数与target最接近，并把它存入差值
                    if(Math.abs(num - target) < Math.abs(dif - target)) dif = num
                    //如果三数之和比target小，左指针右移
                    if(num < target) left ++
                    //如果三数之和比target大，右指针左移
                    else if(num > target) right --
                    else return num
                  
                }
            }
            return dif
        }