func findTargetSumWays(nums []int, S int) (ans int) {
	dfs(nums, len(nums), 0, 0, S, &ans)
	return
}

func dfs(nums []int, n int, i int, sum int, S int, ans *int) {
	if i == n {
		if sum == S {
			(*ans)++
		}
		return
	}
	dfs(nums, n, i+1, sum+nums[i], S, ans)
	dfs(nums, n, i+1, sum-nums[i], S, ans)
}