986. Interval List Intersections

You are given two lists of closed intervals, firstList and secondList, where firstList[i] = [starti, endi] and secondList[j] = [startj, endj]. Each list of intervals is pairwise disjoint and in sorted order.

Return the intersection of these two interval lists.

A closed interval [a, b] (with a <= b) denotes the set of real numbers x with a <= x <= b.

The intersection of two closed intervals is a set of real numbers that are either empty or represented as a closed interval. For example, the intersection of [1, 3] and [2, 4] is [2, 3].

设置两个指针，分别指向两个intervals的头部。
循环，相交的left等于两者左端的较大值。right等于两者右端的较小值。
只有在left小于right时，两个interval才相交，填入列表。
然后更新两个interval中右端较小的指针。

class Solution {
    public int[][] intervalIntersection(int[][] firstList, int[][] secondList) {
        List<int[]> ans = new ArrayList(); 
        int i = 0;
        int j = 0;
        
        while(i < firstList.length && j < secondList.length){
            int left = Math.max( firstList[i][0], secondList[j][0] );
            int right = Math.min( firstList[i][1], secondList[j][1] );
            if(left <= right){
                ans.add(new int[]{left, right});
            }
            if(firstList[i][1] < secondList[j][1]) i++;
            else j++;

        }
        
        int[][] ret = new int[ans.size()][2];
        ans.toArray(ret);
        return ret;
    }
}

class Solution {
    public int[][] intervalIntersection(int[][] firstList, int[][] secondList) {
        int i = 0;
        int j = 0;
        ArrayList<int[]> ans = new ArrayList(); 
        int[] holdA;
        int[] holdB;

        while(i < firstList.length && j < secondList.length){
            holdA = firstList[i];
            holdB = secondList[j];
            
            int[] arr = new int[2];
            
            if(holdA[0] <= holdB[0] && holdA[1] >= holdB[1]){
                ans.add(holdB);
                j++;
            }
            else if(holdA[0] >= holdB[0] && holdA[1] <= holdB[1]){
                ans.add(holdA);
                i++;
            }
            else if(holdA[0] <= holdB[0] && holdA[1] >= holdB[0]){
                arr[0] = holdB[0];
                arr[1] = holdA[1];
                ans.add(arr);
                i++;
            }
            else if(holdA[0] >= holdB[0] && holdB[1] >= holdA[0]){
                arr[0] = holdA[0];
                arr[1] = holdB[1];
                ans.add(arr);
                j++;
            }
            else if(holdA[1] <= holdB[1]){
                i++;
            }
            else{
                j++;
            }
        }
        int[][] ret = new int[ans.size()][2];
        ans.toArray(ret);
        return ret;
    }
}