hdu 3999 The order of a Tree

#include<stdio.h>
#include<string.h>
struct abc{
int left;
int right;
int date;
}node[100010];
int flag[100010];
int ans[100010];
int n, tot;
void dfs(int wei)
{
if (node[wei].left != -1 && flag[node[wei].left] ==0)
{
flag[node[wei].left] = 1;
ans[tot] = node[node[wei].left].date;
tot++;
dfs(node[wei].left);
}
if (node[wei].right != -1 && flag[node[wei].right] == 0)
{
flag[node[wei].right] = 1;
ans[tot] = node[node[wei].right].date;
tot++;
dfs(node[wei].right);
}
}
int main()
{
int i,  k;
while (~scanf("%d", &n))
{
memset(flag, 0, sizeof(flag));
for (i = 0; i <= n; i++) node[i].date = -1, node[i].left = -1, node[i].right = -1;
scanf("%d", &k);
tot = 1;
node[1].date = k;
int nodesum = 2;
for (i = 2; i <= n; i++)
{
scanf("%d", &k);
int nodexun = 1;
while (1)
{
if (k > node[nodexun].date)
{
if (node[nodexun].right == -1)
{
node[nodexun].right = nodesum;
node[nodesum].date = k;
nodesum++;
break;
}
else nodexun = node[nodexun].right;
}
else
{
if (node[nodexun].left == -1)
{
node[nodexun].left = nodesum;
node[nodesum].date = k;
nodesum++;
break;
}
else nodexun = node[nodexun].left;
}
}
}
ans[tot] = node[1].date;
tot++;
flag[1] = 1;
dfs(1);
for (i = 1; i <= n; i++)
{
if (i<n)
printf("%d ", ans[i]);
else printf("%d\n", ans[i]);
}
}
return 0;
}