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| 1 | +/* |
| 2 | + * Deleted code is debugged code :) |
| 3 | + */ |
| 4 | +#include <bits/stdc++.h> |
| 5 | +#define ll long long |
| 6 | +#define ull unsigned long long |
| 7 | +#define vi vector<ll> |
| 8 | +#define pp pair<ll,ll> |
| 9 | +#define mp make_pair |
| 10 | +#define PI acos(-1.0) |
| 11 | +#define all(v) v.begin(),v.end() |
| 12 | +#define pb push_back |
| 13 | +#define FOR(i,a,b) for(i=a;i<b;i++) |
| 14 | +#define FREV(i,a,b) for(i=a;i>=b;i--) |
| 15 | +#define SULL(n) scanf("%llu", &n) |
| 16 | +#define INF 1e18 |
| 17 | +#define MOD 1000000007 |
| 18 | + |
| 19 | +#ifndef ONLINE_JUDGE |
| 20 | +#define gc getchar |
| 21 | +#define pc putchar |
| 22 | +#else |
| 23 | +#define gc getchar_unlocked |
| 24 | +#define pc putchar_unlocked |
| 25 | +#endif |
| 26 | + |
| 27 | +using namespace std; |
| 28 | + |
| 29 | +int read_int() { |
| 30 | + char c = gc(); |
| 31 | + while((c < '0' || c > '9') && c != '-') c = gc(); |
| 32 | + int ret = 0, neg = 0; |
| 33 | + if (c == '-') neg = 1, c = gc(); |
| 34 | + while(c >= '0' && c <= '9') { |
| 35 | + ret = 10 * ret + c - 48; |
| 36 | + c = gc(); |
| 37 | + } |
| 38 | + return neg ? -ret : ret; |
| 39 | +} |
| 40 | + |
| 41 | +ll read_ll() { |
| 42 | + char c = gc(); |
| 43 | + while((c < '0' || c > '9') && c != '-') c = gc(); |
| 44 | + ll ret = 0; |
| 45 | + int neg = 0; |
| 46 | + if (c == '-') neg = 1, c = gc(); |
| 47 | + while(c >= '0' && c <= '9') { |
| 48 | + ret = 10 * ret + c - 48; |
| 49 | + c = gc(); |
| 50 | + } |
| 51 | + return neg ? -ret : ret; |
| 52 | +} |
| 53 | + |
| 54 | +/*******************************************RANDOM STUFF BEGINS HERE**************************************************/ |
| 55 | + |
| 56 | +struct node { |
| 57 | + vi adj; |
| 58 | +}; |
| 59 | + |
| 60 | +vector<node> tree; |
| 61 | +vi depth(100005), parent(100005), section_ancestor(100005); |
| 62 | +ll maxdepth = 0; |
| 63 | +// DFS to calculate depths of each node in O(n) |
| 64 | +void dfs(ll u) { |
| 65 | + ll i, len = tree[u].adj.size(); |
| 66 | + FOR(i,0,len) { |
| 67 | + ll v = tree[u].adj[i]; |
| 68 | + depth[v] = depth[u] + 1; |
| 69 | + if (depth[v] > maxdepth) { |
| 70 | + maxdepth = depth[v]; |
| 71 | + } |
| 72 | + dfs(v); |
| 73 | + } |
| 74 | +} |
| 75 | + |
| 76 | +// LCA preprocessing in O(n) |
| 77 | +// Divide the tree in sqrt(H) sections where H = height of the tree |
| 78 | +// section_ancestor[i] stores the ancestor of node i in the last level of upper section |
| 79 | +void LCA_preprocessing(ll n) { |
| 80 | + ll i, sqrtH = int(sqrt(maxdepth)); |
| 81 | + |
| 82 | + FOR(i,1,n+1) { |
| 83 | + if (i < sqrtH) { |
| 84 | + section_ancestor[i] = 1; |
| 85 | + } |
| 86 | + else if (depth[i] % sqrtH == 0) { |
| 87 | + section_ancestor[i] = parent[i]; |
| 88 | + } |
| 89 | + else { |
| 90 | + section_ancestor[i] = section_ancestor[parent[i]]; |
| 91 | + } |
| 92 | + } |
| 93 | +} |
| 94 | + |
| 95 | +// Returns LCA of x and y in O(sqrt(n)) |
| 96 | +ll getLCA(ll x, ll y) { |
| 97 | + cout << "x = "<< x <<" y = "<< y << endl; |
| 98 | + while (section_ancestor[x] != section_ancestor[y]) { |
| 99 | + if (depth[x] > depth[y]) { |
| 100 | + x = section_ancestor[x]; |
| 101 | + } |
| 102 | + else { |
| 103 | + y = section_ancestor[y]; |
| 104 | + } |
| 105 | + } |
| 106 | + |
| 107 | + // x and y are now in the same section |
| 108 | + while (x != y) { |
| 109 | + if (depth[x] > depth[y]) { |
| 110 | + x = parent[x]; |
| 111 | + } |
| 112 | + else { |
| 113 | + y = parent[y]; |
| 114 | + } |
| 115 | + } |
| 116 | + |
| 117 | + return x; |
| 118 | +} |
| 119 | + |
| 120 | +int main(int argc, char *argv[]) { |
| 121 | + ll i,j,t,n,x,y; |
| 122 | + n = read_ll(); |
| 123 | + tree.resize(n+1); |
| 124 | + // 1 is the root. Accept parents of the remaining (n-1) nodes |
| 125 | + FOR(i,2,n+1) { |
| 126 | + j = read_ll(); |
| 127 | + tree[j].adj.pb(i); |
| 128 | + parent[i] = j; |
| 129 | + } |
| 130 | + dfs(1); |
| 131 | + // cout << "DFS complete ----> maxdepth = " << maxdepth << endl; |
| 132 | + LCA_preprocessing(n); |
| 133 | + // cout << "LCA preprocessing done" << endl; |
| 134 | + // FOR(i,1,n+1) { |
| 135 | + // cout << "section_ancestor[" << i <<"] = " << section_ancestor[i] << endl; |
| 136 | + // } |
| 137 | + x = read_ll(); |
| 138 | + y = read_ll(); |
| 139 | + // cout << "LCA of " << x << " and " << y << " = " << getLCA(x,y) << endl; |
| 140 | + return 0; |
| 141 | +} |
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