414. Third Maximum Number

TreeSet

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class Solution {
    public int thirdMax(int[] nums) {
        TreeSet<Integer> set = new TreeSet<>();
        for (int num : nums) {
            set.add(num);
            if (set.size() > 3) {
                set.remove(set.first());
            }
        }

        return set.size() == 3 ? set.first() : set.last();
    }
}

一次遍历

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class Solution {
    public int thirdMax(int[] nums) {
        Integer a = null, b = null, c = null; // a > b > c
        for (int num : nums) {
            if (a == null || num > a) {
                c = b;
                b = a;
                a = num;
            } else if (num < a && (b == null || num > b)) {
                c = b;
                b = num;
            } else if ((b != null && num < b) && (c == null || num > c)) {
                c = num;
            }
        }

        return c == null ? a : c;
    }
}

References

414. Third Maximum Number