354. 俄罗斯套娃信封问题

输入：envelopes = [[5,4],[6,4],[6,7],[2,3]]
输出：3
解释：最多信封的个数为 3, 组合为: [2,3] => [5,4] => [6,7]。

func maxEnvelopes(envelopes [][]int) int {
    sort.Slice(envelopes, func(i, j int) bool {
        return envelopes[i][0] < envelopes[j][0] || (envelopes[i][0] == envelopes[j][0] && envelopes[i][1] > envelopes[j][1])
    })
    var ans int
    dp := make([]int, len(envelopes))
    for i := range envelopes {
        dp[i] = 1
        for j := 0; j < i; j++ {
            if envelopes[i][1] > envelopes[j][1] {
                dp[i] = max(dp[i], dp[j]+1)
            }
        }
        ans = max(ans, dp[i])
    }
    return ans
}

func max(a, b int) int {
    if a > b {
        return a
    }
    return b
}