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top-100-liked/16_test.go

@@ -0,0 +1,57 @@
+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}))
+}