LC.P238[除自身以外数组的乘积]

方法一：前缀与后缀乘

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class Solution {
    public int[] productExceptSelf(int[] nums) {
        int n = nums.length;
        int[] ans = new int[n], left = new int[n], right = new int[n];
        left[0] = right[n - 1] = 1;
        for (int i = 1; i < n; ++i) left[i] = left[i - 1] * nums[i - 1];
        for (int i = n - 2; i >= 0; --i) right[i] = right[i + 1] * nums[i + 1];
        for (int i = 0; i < n; ++i) ans[i] = left[i] * right[i];
        return ans;
    }
}
  • 时间复杂度：$O(n)$
  • 空间复杂度：$O(n)$

方法二：空间优化

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class Solution {
    public int[] productExceptSelf(int[] nums) {
        int n = nums.length;
        int[] ans = new int[n];
        for (int i = 1, prod = 1; i <= n; ++i) {
            ans[i - 1] = prod;
            prod *= nums[i - 1];
        }
        for (int i = n, prod = 1; i >= 1; --i) {
            ans[i - 1] *= prod;
            prod *= nums[i - 1];
        }
        return ans;
    }
}
  • 时间复杂度：$O(n)$
  • 空间复杂度：$O(1)$