const permuteUnique = function(nums) {
    const len = nums.length, res = [], used = []
    nums.sort((a, b) => a - b)
    const backtrack = (deepStack) => {
        if (deepStack.length === len) {
            res.push(deepStack.slice())
            return
        }
        for (let i = 0; i < len; i++) {
            // 当前选项与上一项相同、且上一项存在、且没有被使用过，则忽略
            if (nums[i - 1] === nums[i] && i - 1 >= 0 && !used[i - 1]) continue 
            if (used[i]) continue // 使用过便不再使用
            deepStack.push(nums[i])
            used[i] = true
            backtrack(deepStack)
            deepStack.pop()
            used[i] = false
        }
    }
    backtrack([])
    return res
}