核心在于子序列不连续，子串连续

if(a[i]== a[j]){

dp[i][j] = dp[i-1][j-1]+1

}

else {

//子串

dp[i][j]=0;

//子序列

dp[i][j]=max(dp[i-1][j],dp[i][j-1])

https://leetcode-cn.com/problems/longest-common-subsequence/