27. Remove Element

Two Pointers

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class Solution {
    private void swap(int[] nums, int i, int j) {
        int tmp = nums[i];
        nums[i] = nums[j];
        nums[j] = tmp;
    }

    public int removeElement(int[] nums, int val) {
        // 将 val 全部交换到最后
        int nonValIndex = nums.length - 1;

        for (int i = nums.length - 1; i >= 0; i--) {
            if (nums[i] == val) {
                swap(nums, i, nonValIndex--);
            }
        }

        return nonValIndex + 1;
    }
}

Two Pointers

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class Solution {
    public int removeElement(int[] nums, int val) {
        int i = 0;
        for (int j = 0; j < nums.length; j++) {
            if (nums[j] != val) {
                nums[i++] = nums[j];
            }
        }

        return i;
    }
}

Two Pointers

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class Solution {
    public int removeElement(int[] nums, int val) {
        int i = 0, j = nums.length - 1;
        while (i <= j) {
            if (nums[i] == val) {
                nums[i] = nums[j--]; // 注意此处没有 i++，因为 nums[i] 可能依然为 val
            } else {
                i++;
            }
        }

        return i;
    }
}

References

27. Remove Element