977. Squares of a Sorted Array 

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class Solution {
    public int[] sortedSquares(int[] nums) {
        int[] res = new int[nums.length];

        int resIndex = res.length - 1;
        int i = 0, j = nums.length - 1;
        // 注意包含等于，否则最后一个数字无法写入 res 中
        while (i <= j) {
            int leftSquare = nums[i] * nums[i];
            int rightSquare = nums[j] * nums[j];

            if (leftSquare > rightSquare) {
                res[resIndex--] = leftSquare;
                i++;
            } else {
                res[resIndex--] = rightSquare;
                j--;
            }
        }

        return res;
    }
}

关键之处在于逆序处理。

References

977. Squares of a Sorted Array