[LeeCode][动态规划][简单] 杨辉三角

class Solution {
    public List<List<Integer>> generate(int numRows) {
        List<List<Integer>> result = new ArrayList<>();
        if(numRows == 1) {
            result.add(Arrays.asList(1));
           return result;
        }
        if(numRows == 2) {
            result.add(Arrays.asList(1));
            result.add(Arrays.asList(1,1));
            return result;
        }
        // start 3
        List<List<Integer>> beforeRows = generate(numRows - 1);
        List<Integer> curRow = getCurRow(beforeRows);
        beforeRows.add(curRow);
        return beforeRows;
    }
    public List<Integer> getCurRow(List<List<Integer>> beforeRows){
        // 取最后一行 计算当前行
        List<Integer> line = beforeRows.get(beforeRows.size() -1);
        List<Integer> curRow = new ArrayList<>();
        curRow.add(1);
        for (int i = 0;i < line.size() - 1; i++) {
            curRow.add(line.get(i) + line.get(i + 1));
        }
        curRow.add(1);
        return curRow;
    }
}