[Link] https://www.acmicpc.net/problem/2169
Point:
Have to initialize left_dp[0] to -INF
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#include <algorithm>
#include <iostream>
#include <vector>
// #include <bits/stdc++.h>
using namespace std;
#define for0(i, n) for(int i = 0; i < n; i++)
#define for1(i, n) for(int i = 1; i <= n; i++)
#define fori(s, e) for(int i = s; i < e; i++)
typedef long long ll;
const int INF = 987654321;
int n, m, ans = INF;
int mm[1000][1000];
int left_dp[1000][1000];
int right_dp[1000][1000];
void solve() {
right_dp[0][0] = mm[0][0];
left_dp[0][0] = -INF;
for1(i, m) {
right_dp[0][i] = right_dp[0][i-1] + mm[0][i];
left_dp[0][i] = -INF;
}
for1(i, n) {
right_dp[i][0] = max(left_dp[i-1][0], right_dp[i-1][0]) + mm[i][0];
for1(j, m) {
right_dp[i][j] = max({ left_dp[i-1][j], right_dp[i-1][j], right_dp[i][j-1] }) + mm[i][j];
}
left_dp[i][m-1] = max(left_dp[i-1][m-1], right_dp[i-1][m-1]) + mm[i][m-1];
for(int j = m - 2; j >= 0; j--) {
left_dp[i][j] = max({ left_dp[i-1][j], right_dp[i-1][j], left_dp[i][j+1] }) + mm[i][j];
}
}
}
int main() {
ios_base::sync_with_stdio(0);cin.tie(0); cout.tie(0);
cin >> n >> m;
for0(i, n) {
for0(j, m) {
cin >> mm[i][j];
}
}
solve();
cout << max(left_dp[n-1][m-1], right_dp[n-1][m-1]);
return 0;
}