package top100liked

import (
	"fmt"
	"testing"
)

// https://leetcode.cn/problems/product-of-array-except-self/?envType=study-plan-v2&envId=top-100-liked

// func productExceptSelf(nums []int) []int {
// 	left := make([]int, len(nums))
// 	right := make([]int, len(nums))
// 	left[0] = nums[0]
// 	right[len(nums)-1] = nums[len(nums)-1]
// 	for i := 1; i < len(nums); i++ {
// 		left[i] = left[i-1] * nums[i]
// 	}
// 	for i := len(nums) - 2; i >= 0; i-- {
// 		right[i] = right[i+1] * nums[i]
// 	}
// 	res := []int{}
// 	for i := range nums {
// 		if i == 0 {
// 			res = append(res, right[1])
// 		} else if i == len(nums)-1 {
// 			res = append(res, left[len(nums)-2])
// 		} else {
// 			res = append(res, left[i-1]*right[i+1])
// 		}
// 	}
// 	return res
// }

func productExceptSelf(nums []int) []int {
	n := len(nums)
	answer := make([]int, n)

	// 第一遍：计算左边乘积，存入 answer
	answer[0] = 1
	for i := 1; i < n; i++ {
		answer[i] = answer[i-1] * nums[i-1]
	}

	// 第二遍：从右向左，用 R 维护右边乘积
	R := 1
	for i := n - 1; i >= 0; i-- {
		answer[i] = answer[i] * R // 左边积 × 右边积
		R = R * nums[i]           // 更新右边积
	}

	return answer
}

func TestS16(t *testing.T) {
	fmt.Printf("%+v", productExceptSelf([]int{1, 2, 3, 4}))
	fmt.Printf("%+v", productExceptSelf([]int{-1, 1, 0, -3, 3}))
}