Solution.java

class Solution {
    public int[][] outerTrees(int[][] trees) {
        int n = trees.length;
        if (n < 4) {
            return trees;
        }
        Arrays.sort(trees, (a, b) -> { return a[0] == b[0] ? a[1] - b[1] : a[0] - b[0]; });
        boolean[] vis = new boolean[n];
        int[] stk = new int[n + 10];
        int cnt = 1;
        for (int i = 1; i < n; ++i) {
            while (cnt > 1 && cross(trees[stk[cnt - 1]], trees[stk[cnt - 2]], trees[i]) < 0) {
                vis[stk[--cnt]] = false;
            }
            vis[i] = true;
            stk[cnt++] = i;
        }
        int m = cnt;
        for (int i = n - 1; i >= 0; --i) {
            if (vis[i]) {
                continue;
            }
            while (cnt > m && cross(trees[stk[cnt - 1]], trees[stk[cnt - 2]], trees[i]) < 0) {
                --cnt;
            }
            stk[cnt++] = i;
        }
        int[][] ans = new int[cnt - 1][2];
        for (int i = 0; i < ans.length; ++i) {
            ans[i] = trees[stk[i]];
        }
        return ans;
    }

    private int cross(int[] a, int[] b, int[] c) {
        return (b[0] - a[0]) * (c[1] - b[1]) - (b[1] - a[1]) * (c[0] - b[0]);
    }
}