题解 | 校门外的树

shu,n = map(int,input().split())
lu=[]
for i in range (n):
    l,r =map(int,input().split())
    lu.append([l,r])
lu.sort(key=lambda x: x[0])
colu=[lu[0]]
for cur in lu[1:]:
    last = colu[-1]
    if cur[0]<=last[1]:
        colu[-1]=[last[0],max(last[1],cur[1])]
    else:
        colu.append(cur)
#print(colu)
total=0
for i in range(len(colu)):
    total+=colu[i][1]-colu[i][0]+1
print(shu+1-total)  
    

区间覆盖处理思路